Oil In Expansion Tank

[gunman]

Hi all, new to this forum but have been into audis for a couple of years.

Problem is this. I have a 93 audi 1.9 tdi and I have noticed lately some oil (scum) floating in the expansion tank. There is no white smoke in the exhaust and the water system doesnt build any abnormal pressure. Doesnt seem like a head gasket although the engine does use some water but always has. Oh, also the oil in the engine is clean with no water contamination.

So I'm wondering, where would it be coming from and is it serious? Is there an oil cooler? Is this a common fault?

Any help greatly appreciated.

Cheers