Oil Viscosity FAQ -- Please comment
11-17-1999, 04:25 PM
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Oil Viscosity FAQ -- Please comment
Oil Viscosity FAQ
The question of which viscosity grade of motor oil to use in newer Audi engines seems to come up daily on the forums. Herein I will attempt an answer:
Any 5W-30 motor oil, conventional or synthetic, displaying the API (American Petroleum Institute) “starburst” on the front of the container should be sufficient for all seasons, winter and summer, even with very hard street driving. That’s the short answer, those not interested in details need read no further. Others, read on.
All metal engine parts need to be separated from each other while moving in order to prevent friction and wear on the parts. This in done in modern production engines using a thin film of oil (usually less than 1/10,000 of an inch thick). Oil’s viscosity, in part, determines how thick a film the oil will form between moving parts, and how much friction there will be between those parts when separated by the oil. Higher viscosity means thicker oil films and more friction within the oil. Obviously, one wants a viscosity high enough to prevent parts from grinding against each other, but no higher than that, because any more results in excessive friction within the oil, leading to excess heat and power loss. The ideal oil would provide the minimum required viscosity under all conditions. Unfortunately, oil’s viscosity diminishes with increasing temperature, necessitating the use of an oil which will have the minimum required viscosity at the highest expected temperature, and greater than optimal viscosity at lower temperatures.
The first number, preceding the ‘W’ (for Winter), in an oil’s viscosity grade, is an indication of low-temperature performance -- the lower the better. The second number represents the oil’s high-temperature viscosity, but higher is not always better! One wants to choose the minimum viscosity that results in an oil film thick enough to prevent wear at the temperatures one expects to encounter. Only the engineers that designed the engine know what viscosity is required to provide adequate film thickness without unnecessary friction, so you’re stuck with their recommendation. For what it’s worth, I trust the engineers -- Volkswagen warrants it’s engines for 100,000 miles, which means that it must expect few failures before 100,000 miles, even given most users tendency to ignore proper break-in, warm-up, oil-drain intervals, and maintenance procedures. So, if you plan on getting rid of your car before 100,000 miles, stop reading here. Any conventional 5W-30, as used by many VW and Audi dealers, will not affect your engine life before 100,000 miles. People who intend to keep their cars for 200,000 miles or more, like myself, continue reading.
Older owner’s manuals showed a chart indicating that “energy conserving” oils bearing the API starburst in both viscosity grades 5W-30 and 10W-30 were suitable for all temperatures, but a confusing note indicated that 5W-30 oils should not be used for “high-speed, long-distance driving.” Beginning with the 2000 model year, manuals indicate that any conventional or synthetic motor oil may be used as long as it meets one of the following specifications: ACEA A2 or A3, VW 500.00 or 505.00, or API SJ. API SJ is the most current API specification, which, if the oil also meets the most current ILSAC (International Lubricants Standardization and Approval Committee) and energy conserving requirements, is always indicated by a “starburst” on the front label. The manual also notes that 5W-40 is the factory fill oil, and other sources have indicated that it is a conventional mineral oil, not synthetic. The manual also states that 5W-30 may be used if 5W-40 is not available. 5W-40 is a viscosity grade most commonly found in Europe, as are the ACEA specifications. (ACEA is the European automobile manufacturers association, which includes Volkswagen.)
There has been some concern expressed among new owners that 5W-40 is not more available in the U.S. An API SJ approved 5W-30 must have a high-temperature/high-shear (HTHS) viscosity of at least 2.9 centiPoise (cP), and most brands, conventional and synthetic, fall within the range of 2.9 to 3.5 cP. This is lower than the minimum 3.5 cP required of ACEA A2 and A3 oils, and many have wondered if 5W-30 is a compromise that they shouldn’t be making with their expensive new cars. (I wondered that myself.) However, it seems likely that Audi recommends 5W-40 where available, because most places where it is available tend to have the possibility of “high-speed, long distance driving,” which, in the older owner’s manuals, you are instructed to avoid while using 5W-30.
“High-speed, long distance driving,” by European standards, likely means several tens of miles at speeds above 100 mph. The power required overcome atmospheric drag at 130 mph (most Audi’s governed top-speed) is more than twice that required at 100 mph. The result of this is that at any average speed of less than 100 mph, your engine is under only half the load and is probably producing less than half the heat that Audi felt necessitated a 5W-40 oil. Driving at 75 mph in an Audi (which generally have excellent aerodynamics and low drag) requires only about 25 hp, and does not heavily load the engine. Your engine is likely more than well protected with an API SJ 5W-30 if you drive mainly on public roads, and do not average speeds greater than 100 mph over distances greater than a few tens of miles. Short sprints up to 130, if you are fortunate enough to live where that is possible, are not to be feared. Only sustained high-speeds heat the oil enough to require the higher viscosity specified by ACEA A2 or A3. Many people have reported that even at local track events their oil temperature does not increase dramatically. Therefore, the only conditions which might require 5W-40 are likely those which result in seriously heavy loading of the engine, such as prolonged high-speed, or towing a heavy trailer up the side of a mountain. (It is not coincidence that the primary users of 5W- and 15W-40 oils in the U.S. are operators of tractor-trailer fleets!) Given that, it is almost certain that higher viscosity oils, such as 5W- and 15W-50, are complete wastes of energy, placing undue strain on your engine, and raising operating temperatures for no real benefit.
Thus far I have written mostly about 5W-30, and avoided other viscosity grades, and the contentious issue of conventional versus synthetic oil. 0W-, 5W-, and 10W-30 oils all must meet the same 2.9 cP HTHS viscosity requirement for API SJ certification, and therefore will all provide adequate protection for your engine. The lower winter viscosity grades will be more valuable in colder climates. 10W generally flows and pumps fine at four degrees below zero on the Fahrenheit scale, and each grade below that buys you another nine degrees. Synthetic oil is vastly superior to conventional oil in its low-temperature pumpability, high-temperature stability, long-drain capability, and high lubricity (low friction). However, all these benefits may not be apparent in a car used in a temperate climate, with oil changes as specified by the owner’s manual (5000, 7500, or 10,000 miles). I therefore cannot say if you will reap enough benefit to recoup the added costs of changing your oil with synthetic. I live near Boston, and replace the oil in my 1.8T with Mobil 1 5W-30 at the required 5,000 mile drain interval while under warranty.
As always, YMMV.
The question of which viscosity grade of motor oil to use in newer Audi engines seems to come up daily on the forums. Herein I will attempt an answer:
Any 5W-30 motor oil, conventional or synthetic, displaying the API (American Petroleum Institute) “starburst” on the front of the container should be sufficient for all seasons, winter and summer, even with very hard street driving. That’s the short answer, those not interested in details need read no further. Others, read on.
All metal engine parts need to be separated from each other while moving in order to prevent friction and wear on the parts. This in done in modern production engines using a thin film of oil (usually less than 1/10,000 of an inch thick). Oil’s viscosity, in part, determines how thick a film the oil will form between moving parts, and how much friction there will be between those parts when separated by the oil. Higher viscosity means thicker oil films and more friction within the oil. Obviously, one wants a viscosity high enough to prevent parts from grinding against each other, but no higher than that, because any more results in excessive friction within the oil, leading to excess heat and power loss. The ideal oil would provide the minimum required viscosity under all conditions. Unfortunately, oil’s viscosity diminishes with increasing temperature, necessitating the use of an oil which will have the minimum required viscosity at the highest expected temperature, and greater than optimal viscosity at lower temperatures.
The first number, preceding the ‘W’ (for Winter), in an oil’s viscosity grade, is an indication of low-temperature performance -- the lower the better. The second number represents the oil’s high-temperature viscosity, but higher is not always better! One wants to choose the minimum viscosity that results in an oil film thick enough to prevent wear at the temperatures one expects to encounter. Only the engineers that designed the engine know what viscosity is required to provide adequate film thickness without unnecessary friction, so you’re stuck with their recommendation. For what it’s worth, I trust the engineers -- Volkswagen warrants it’s engines for 100,000 miles, which means that it must expect few failures before 100,000 miles, even given most users tendency to ignore proper break-in, warm-up, oil-drain intervals, and maintenance procedures. So, if you plan on getting rid of your car before 100,000 miles, stop reading here. Any conventional 5W-30, as used by many VW and Audi dealers, will not affect your engine life before 100,000 miles. People who intend to keep their cars for 200,000 miles or more, like myself, continue reading.
Older owner’s manuals showed a chart indicating that “energy conserving” oils bearing the API starburst in both viscosity grades 5W-30 and 10W-30 were suitable for all temperatures, but a confusing note indicated that 5W-30 oils should not be used for “high-speed, long-distance driving.” Beginning with the 2000 model year, manuals indicate that any conventional or synthetic motor oil may be used as long as it meets one of the following specifications: ACEA A2 or A3, VW 500.00 or 505.00, or API SJ. API SJ is the most current API specification, which, if the oil also meets the most current ILSAC (International Lubricants Standardization and Approval Committee) and energy conserving requirements, is always indicated by a “starburst” on the front label. The manual also notes that 5W-40 is the factory fill oil, and other sources have indicated that it is a conventional mineral oil, not synthetic. The manual also states that 5W-30 may be used if 5W-40 is not available. 5W-40 is a viscosity grade most commonly found in Europe, as are the ACEA specifications. (ACEA is the European automobile manufacturers association, which includes Volkswagen.)
There has been some concern expressed among new owners that 5W-40 is not more available in the U.S. An API SJ approved 5W-30 must have a high-temperature/high-shear (HTHS) viscosity of at least 2.9 centiPoise (cP), and most brands, conventional and synthetic, fall within the range of 2.9 to 3.5 cP. This is lower than the minimum 3.5 cP required of ACEA A2 and A3 oils, and many have wondered if 5W-30 is a compromise that they shouldn’t be making with their expensive new cars. (I wondered that myself.) However, it seems likely that Audi recommends 5W-40 where available, because most places where it is available tend to have the possibility of “high-speed, long distance driving,” which, in the older owner’s manuals, you are instructed to avoid while using 5W-30.
“High-speed, long distance driving,” by European standards, likely means several tens of miles at speeds above 100 mph. The power required overcome atmospheric drag at 130 mph (most Audi’s governed top-speed) is more than twice that required at 100 mph. The result of this is that at any average speed of less than 100 mph, your engine is under only half the load and is probably producing less than half the heat that Audi felt necessitated a 5W-40 oil. Driving at 75 mph in an Audi (which generally have excellent aerodynamics and low drag) requires only about 25 hp, and does not heavily load the engine. Your engine is likely more than well protected with an API SJ 5W-30 if you drive mainly on public roads, and do not average speeds greater than 100 mph over distances greater than a few tens of miles. Short sprints up to 130, if you are fortunate enough to live where that is possible, are not to be feared. Only sustained high-speeds heat the oil enough to require the higher viscosity specified by ACEA A2 or A3. Many people have reported that even at local track events their oil temperature does not increase dramatically. Therefore, the only conditions which might require 5W-40 are likely those which result in seriously heavy loading of the engine, such as prolonged high-speed, or towing a heavy trailer up the side of a mountain. (It is not coincidence that the primary users of 5W- and 15W-40 oils in the U.S. are operators of tractor-trailer fleets!) Given that, it is almost certain that higher viscosity oils, such as 5W- and 15W-50, are complete wastes of energy, placing undue strain on your engine, and raising operating temperatures for no real benefit.
Thus far I have written mostly about 5W-30, and avoided other viscosity grades, and the contentious issue of conventional versus synthetic oil. 0W-, 5W-, and 10W-30 oils all must meet the same 2.9 cP HTHS viscosity requirement for API SJ certification, and therefore will all provide adequate protection for your engine. The lower winter viscosity grades will be more valuable in colder climates. 10W generally flows and pumps fine at four degrees below zero on the Fahrenheit scale, and each grade below that buys you another nine degrees. Synthetic oil is vastly superior to conventional oil in its low-temperature pumpability, high-temperature stability, long-drain capability, and high lubricity (low friction). However, all these benefits may not be apparent in a car used in a temperate climate, with oil changes as specified by the owner’s manual (5000, 7500, or 10,000 miles). I therefore cannot say if you will reap enough benefit to recoup the added costs of changing your oil with synthetic. I live near Boston, and replace the oil in my 1.8T with Mobil 1 5W-30 at the required 5,000 mile drain interval while under warranty.
As always, YMMV.
11-17-1999, 05:38 PM
#4
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looks good: there was a mention of 15W50, btw (to Sean T)... [m]
you outlined well the reason I've just decided to stop using 15W50 in summer & 10W30 winter (Mobil 1) in my 98.5 2.8: I'm convinced there's no need in a non turbo for 15W50 under normal use.
11-17-1999, 06:54 PM
#5
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Just a comment from an A4 owner's manual
I'm not an engineer, just a technician, so my only reference is my 1999 A4 manual.
The note in my manual has a little more information:
"When using SAE 5 W-30 engine oil, avoid high-speed, long-distance driving if outside temperature rises above the indicated limits."
The indicated limits for an energy conserving oil (since that is what was mentioned) is approximately 95 degrees Fahrenheit or 38 degrees Celsius (as read on the chart, not by mathematical conversion).
Charles Noroña
1999.5 A4 1.8T Avant < 2000 miles
The note in my manual has a little more information:
"When using SAE 5 W-30 engine oil, avoid high-speed, long-distance driving if outside temperature rises above the indicated limits."
The indicated limits for an energy conserving oil (since that is what was mentioned) is approximately 95 degrees Fahrenheit or 38 degrees Celsius (as read on the chart, not by mathematical conversion).
Charles Noroña
1999.5 A4 1.8T Avant < 2000 miles
11-17-1999, 08:18 PM
#6
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oil consumption
A side note:
A friend of mine drives a 1993 Audi 90 CS, which has the same 12V 2.8L V6 engine as the 1996 and 1997 A4 2.8. He tried using 10W-30 for a while but found his oil consumption was noticeably greater with the lighter oil- it was immeasurable once he started using 15W-50.
Oil consumption is typical of all engines, but this was evidence enough to me that there is a difference between 15-50 and 10W-30.
Happy Driving,
Peter G
96 12V 2.8 QM
A friend of mine drives a 1993 Audi 90 CS, which has the same 12V 2.8L V6 engine as the 1996 and 1997 A4 2.8. He tried using 10W-30 for a while but found his oil consumption was noticeably greater with the lighter oil- it was immeasurable once he started using 15W-50.
Oil consumption is typical of all engines, but this was evidence enough to me that there is a difference between 15-50 and 10W-30.
Happy Driving,
Peter G
96 12V 2.8 QM
11-17-1999, 09:19 PM
#7
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Re: Oil Viscosity FAQ -- Please comment
Nice post. I am not as big a fan of starburst 5-30 conventional oils. OK if you drive easy and change often, but they have higher VI's than the 10-40's that many car mfg's outlawed, since the excessive vi improvers tended to break down if used hard (and not auto-baun 'hard' for US mfgs). I do like conv'l 10-30 with lower VI's, and synthetic 5-30 ( or semi's if the numbers look good). It is my understanding that VI is tested at the same two temps for the different visc grades, correct me if wrong.
As you noted, a prefered visc is desired to establish a dynamic film when running, and this is about 10-15 cSt for conv oil, based on 212 f props/typ oil temp. Problem is the stuff can be several orders of magnitude higher than this in cold weather starts, if the -40 and -50 oils are used, causing excessive drag, a lot of pressure relief, and not great lubrication. I use a -50 oil in my track (driver-ed..) car, but only when temps are near 90F, and only at the track.
Something is weird on the ACEA A2 A3 req'ts for Y2000. API SJ includes sub 3.5 HTHS oils, and you note A2 and A3 are the HTHS oils, and either is fine with VW...why did they not also include the 'A1?' low shear oils like 10-30? Mabe A2 is low shear? And the 5-40, 0-40, and 10-40 are not necessarily HSHT grades. Like any xx-30, they have a minimum Shear rating of 2.9 units. 15-40, 20-40, 25-40, and xx-50 all are HSHT.
In general, a 5-xx pumps better at -13F than a 10-xx oil. But as I showed below on the Castrol thread, these specs have wide ranges, and Mobil-1 10-30 will flow as well at -13F as the Castrol 5-50, even though the 10-xx official low rating is at -4F. If ****, it's best to look at the specs for each oil.
I prefer M-1 10-30 fr the mid-atlantic area.
Now you need to get your post and other worthy info into a formal FAQ entry, so it can be pointed to in the future. Scanning archives can unearth oil info that is wrong. Good oil info sites could be included.
As you noted, a prefered visc is desired to establish a dynamic film when running, and this is about 10-15 cSt for conv oil, based on 212 f props/typ oil temp. Problem is the stuff can be several orders of magnitude higher than this in cold weather starts, if the -40 and -50 oils are used, causing excessive drag, a lot of pressure relief, and not great lubrication. I use a -50 oil in my track (driver-ed..) car, but only when temps are near 90F, and only at the track.
Something is weird on the ACEA A2 A3 req'ts for Y2000. API SJ includes sub 3.5 HTHS oils, and you note A2 and A3 are the HTHS oils, and either is fine with VW...why did they not also include the 'A1?' low shear oils like 10-30? Mabe A2 is low shear? And the 5-40, 0-40, and 10-40 are not necessarily HSHT grades. Like any xx-30, they have a minimum Shear rating of 2.9 units. 15-40, 20-40, 25-40, and xx-50 all are HSHT.
In general, a 5-xx pumps better at -13F than a 10-xx oil. But as I showed below on the Castrol thread, these specs have wide ranges, and Mobil-1 10-30 will flow as well at -13F as the Castrol 5-50, even though the 10-xx official low rating is at -4F. If ****, it's best to look at the specs for each oil.
I prefer M-1 10-30 fr the mid-atlantic area.
Now you need to get your post and other worthy info into a formal FAQ entry, so it can be pointed to in the future. Scanning archives can unearth oil info that is wrong. Good oil info sites could be included.
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11-18-1999, 12:38 AM
#8
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Great, but both my dealership and my independent here...
...in seattle recommend 10W-40, 15W-40 or 15W-50 for the 2.8 and for older 5 cylinder Audi's (not sure about turbo 5's though), while 10W-30 seems to be the oil of choice for the 1.8T. Being an avid Mobil 1 15W-50 user on a '98 2.8 and an '89, 90Q, I can tell you that the lighter oils result in more valve noise (especially at startup), and consumption. Any way you look at it though, a 15W-50 synthetic is going to have much improved cold flow characteristics than a Dino oil. I have been using Mobil 1 15W-50 for years with no oil related problems. Obviously, in a cold climate, a lighter oil would be a good idea for the cold months.
Charles
Charles
11-18-1999, 04:57 AM
#10
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Thanks for the comments...and here are my replies
I was under the impression that conventional 5W-30 oils use fewer viscosity modifiers than 10W-40 oils, though you are correct that 10W-30 has fewer than 5W-30. I had wondered whether I should touch on that subject. Viscosity Index (VI) is calculated from kinematic viscosity at 40 and 100 degrees Celsius, for all grades. I don't think that VI is necessarily indicative of the amount of viscosity modifiers used though. Someone please post if you know otherwise.
The viscosity at 212 F doesn't really matter, except as an indication of viscosity at higher temps. That's what I meant when I wrote that the ideal oil would provide the same minimum required viscosity (say, 2.6 cP) at all temps.
5W-40 and 5W-50 grade oils won't necessarily be overly thick when cold at startup, that is largely determined by the winter rating. 5W-50 flows just as easily at -13 F as does 5W-30. IIRC, the SAE requires both to display less than 3500 cP in a cold cranking simulator (CCS), and to have less than 60,000 cP viscosity with no yield stress at -22 F. The viscosity of a 10W-30 will be lower (better) at 40 C than a 5W-50, but there you're sort of "inbetween" the 10 and the 30 or the 5 and the 50. This is simply another indication that if you don't need an xW-50 you shouldn't be using one.
A Mobil rep has informed me that all Mobil 1 xW-30 oils are really 0W-30 oils, in that they all meet the 0W spec for low temperature viscosity. Mobil just needed a way to differentiate its oils, and the 0W- and 5W-30 simply exceed the 0W spec by a greater margin than the 10W-30. The 5W- and 10W-30 also have slightly higher HTHS viscosity than the 0W-30 (0W = 3.0 cP, 5W = 3.1 cP, 10W = 3.2 cP). Then there are those who would never buy an oil labled 0W-30, but would sing the praises of the same oil if labled 10W-40. IMO, they're all so similar that I wouldn't care which I was using.
The ACEA A2 and A3 specs require a HTHS viscosity of 3.5 cP, but A3 is a higher quality grade, with stricter requirements mostly relating to long-drain capability. API SJ requires only 2.9 cP of xW-30, monograde 30, 0W-, 5W-, and 10W-40 oils. 3.7 cP is required for 15W-40 and higher. ACEA A1 specifies HTHS between 2.9 and 3.5 cP, and is of the same quality as A3. Again, I believe that VW didn't specify the A1 oils because of the possibility of high-speed, long-distance driving in Europe.
An alternate theory, and one that I have wrestled with, is that in order to obtain the desired 3.5 cP HTHS viscosity in North America, VW would have had to recommend an API SJ 15W-40, which would have been less than optimal for use below about 5 degrees F. Most major brands of 10W-40 actually meet at least the A2 spec, but most manufacturers don't advertise this fact, and most of their telephone tech reps are either uninformed or reluctant to claim A2 or A3 compliance over the phone. So far, the only A3 oils I've found, after an exhaustive search, are Mobil 1 15W-50, Valvoline SynPower 5W-40, and Castrol Syntec 5W-40, which is available exclusively through VW-Audi dealers. It is quite possible that VW simply threw up it's hands and told North American consumers to go with whatever API SJ oil they could find.
The viscosity at 212 F doesn't really matter, except as an indication of viscosity at higher temps. That's what I meant when I wrote that the ideal oil would provide the same minimum required viscosity (say, 2.6 cP) at all temps.
5W-40 and 5W-50 grade oils won't necessarily be overly thick when cold at startup, that is largely determined by the winter rating. 5W-50 flows just as easily at -13 F as does 5W-30. IIRC, the SAE requires both to display less than 3500 cP in a cold cranking simulator (CCS), and to have less than 60,000 cP viscosity with no yield stress at -22 F. The viscosity of a 10W-30 will be lower (better) at 40 C than a 5W-50, but there you're sort of "inbetween" the 10 and the 30 or the 5 and the 50. This is simply another indication that if you don't need an xW-50 you shouldn't be using one.
A Mobil rep has informed me that all Mobil 1 xW-30 oils are really 0W-30 oils, in that they all meet the 0W spec for low temperature viscosity. Mobil just needed a way to differentiate its oils, and the 0W- and 5W-30 simply exceed the 0W spec by a greater margin than the 10W-30. The 5W- and 10W-30 also have slightly higher HTHS viscosity than the 0W-30 (0W = 3.0 cP, 5W = 3.1 cP, 10W = 3.2 cP). Then there are those who would never buy an oil labled 0W-30, but would sing the praises of the same oil if labled 10W-40. IMO, they're all so similar that I wouldn't care which I was using.
The ACEA A2 and A3 specs require a HTHS viscosity of 3.5 cP, but A3 is a higher quality grade, with stricter requirements mostly relating to long-drain capability. API SJ requires only 2.9 cP of xW-30, monograde 30, 0W-, 5W-, and 10W-40 oils. 3.7 cP is required for 15W-40 and higher. ACEA A1 specifies HTHS between 2.9 and 3.5 cP, and is of the same quality as A3. Again, I believe that VW didn't specify the A1 oils because of the possibility of high-speed, long-distance driving in Europe.
An alternate theory, and one that I have wrestled with, is that in order to obtain the desired 3.5 cP HTHS viscosity in North America, VW would have had to recommend an API SJ 15W-40, which would have been less than optimal for use below about 5 degrees F. Most major brands of 10W-40 actually meet at least the A2 spec, but most manufacturers don't advertise this fact, and most of their telephone tech reps are either uninformed or reluctant to claim A2 or A3 compliance over the phone. So far, the only A3 oils I've found, after an exhaustive search, are Mobil 1 15W-50, Valvoline SynPower 5W-40, and Castrol Syntec 5W-40, which is available exclusively through VW-Audi dealers. It is quite possible that VW simply threw up it's hands and told North American consumers to go with whatever API SJ oil they could find.