Tech Article Title Author Date
Engine Oil Viscosity Discussion John Wilkinson 1999

The question of which viscosity grade of motor oil is best for use in newer Audi engines seems to come up daily in the forums. Herein I will attempt an answer:

Any 5W-30 motor oil, conventional or synthetic, displaying the "starburst" API (American Petroleum Institute) certification mark 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 the oil film will be, 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 oils 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 its worth, I trust the engineers -- Volkswagen warrantees 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 I do, may continue reading.

Older owner's manuals showed a chart indicating that "energy conserving" 5W-30 was suitable for all temperatures, but a confusing note indicated that 5W-30 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 Lubricant Standardization and Approval Committee) 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 stands for Association des Constructeurs Europiens de l'Automobile -- the Association of European Automotive Manufacturers, of which Volkswagen is a member.)

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 2.9-3.5 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 allow "high-speed, long distance driving."

"High-speed, long distance driving," by European standards, likely means several tens of miles at speeds above 100 mph. The power required to 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 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, should not cause a problem. Only sustained high-speeds heat the oil enough to significantly reduce viscosity. Many people have reported that even at local track events their oil temperature has not increased dramatically. Therefore, the only conditions which might require 5W-40 are those which result in prolonged heavy loading of the engine, such as "high-speed, long-distance driving," or towing a heavy trailer up the side of a mountain. Given that, it is almost certain that higher viscosity oils, such as 5W- and 15W-50, are complete wastes of horsepower, placing undue (though minor) additional strain on your engine, and raising operating temperatures for no real benefit.

In addition to this reasoning, and Audi's recommendation, I have also received recommendations for 5W-30 from two synthetic lubricant manufacturers -- Mobil and Redline. I spoke with real engineers at both companies, and they were adamant that even for very hard street use in North America, any viscosity grade higher than 30 would be a waste. All felt that I would see the best performance in my 1.8T, especially reduced turbo lag, by using one of their 30 grade oils. Since both companies market higher viscosity oils (10W-40 and 15W-50), neither had any apparent reason to attempt to "sell" me on lower viscosity oil, yet both strongly recommended that I use 30 grade.

Thus far I have written mostly about 5W-30, and ignored other viscosity grades and the contentious issue of conventional versus synthetic oil. All 0W-, 5W-, and 10W-30 oils must meet the same 2.9 cP HTHS viscosity requirement for API SJ certification, and therefore all will provide ample protection for your engine. The lower winter viscosity grades will be more valuable in colder climates. 10W generally flows and pumps well 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 of the same grade 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 at intervals specified by the owners manual. 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 change my oil with Mobil 1 5W-30 at the required 5,000 mile drain intervals while under warranty.

As always, YMMV.

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