N_Jay

White sludge is emulsified water.

Typically a head gasket leak (or cracked head or block).

jcman

Naw that's a bit extreme with the later "typical" problem, the tan butter like substance on the bottom of the oil fill cap or even on the top part of the dip stick that we see is plain old moisture in the air that does not get out of the crankcase because of short cold trips and re-condenses, the pcv system on all C5's has it's short comings with sludge more so caused by moisture.

Then we go back to the pick your poison on oil, synthetic or mineral and which one release's moisture or retains moisture at certain temperatures.

I will agree that if the oil is white or tan in warm/hot seasons then we should be concerned about gaskets or cracks.

N_Jay

OK,
Have not had the A6 long enough, but I have not seen engines with condensation caused oil emulsion since open crank case vents have gone out of use.

jcman

I can't recall ever seeing it in engines with valve cover breathers and I can list a few going back to the late 70's (older cars) all with iron blocks with iron heads running dino oil during cold winters that would include the current non VW/Audi pcv engines too, the most logical reason we see this in VAG engines is in the moisture trap design and the fast engine cool down rate.

I've seen the oil butter on the 01 A6 2.8 our daughter drives and in our 04 A6 3.0 with no apparent issues of oil or coolant loss other than a CCV on the 3.0 that had frozen oil butter in it causing wild idle issues and a loud vacuum hiss, I still replaced the CCV with a new one and this season I'm using Pentosin vs Shell Rotela T6 and the results look better with way less butter.

One point not to dismiss is the pcv system keeps blow-bye pressure at a norm by a vacuum draw, not enough will pressurize the crankcase causing gasket leaks and contaminate the oil with un-burnt gas and water vapor, too much draw or vacuum will cause idle/engine issues and potentially pull oil into the intake, a delicate balance of design by demand so to speak.

old96A4

I don't seem to have any trouble just wondering what the butter was from , sounds like in colder areas you'd get more butter. I use the Mobil 1 euro blend 0-40 synth oil. with the Mann filters. I use to use castrol 10w-40 GTX , the butter looked the same in both cases. Oil looks ok, I change it twice a year. car has 120K miles on it and knock on wood it's been pretty good so far. Just have a few things needing to be looked at but nothing stopping me from driving it .

jcman

The oil butter: It's from the moisture trapped in the high points of PCV system including the valve covers that migrates into the oil or oil sludge "at that point", the moisture is manly a byproduct of the intake air that gets past the piston rings during combustion into the crankcase and this small amount does add-up over time especially during cold damp short trips.

I suppose that members in Arizona or other hot dry states don't have this issue, actually I'm very sure of this.