Yellow foamy stuff on engine oil filler cap...

[FroggleRock]

I had my AUDI A3 1.8SE serviced for its 90K miles.
At the garage, they used fully synthetic Mobil1 5W30.
Later on my girlfriend used this car for a week and when she gave it back to me, told me the oil indicator had gone on . She stopped at the first petrol station..Great so far. Not knowing what oil to use to top-up, she asked around and a guy told her that semi-synthetic oil should do! She added 0.5 litre of the stuff.
Now, I was always told that topping up with not exactly the right oil is not the end of the world... but few hundred miles later I see some yellow pasty stuff on the engine oil cap ..when I remove the cap.
So, obviously I'm a bit concerned....humm... shall I be more than just concerned?

Why on earth is the oil designation such a mess: in the construtor's manual you find designations like VW 500 00, on the oil containers 5W10. 0W30... what this all about?
No blame on my girlfriend of course...I'm the one who should have checked the levels before I gave her the keys.

[Huweth]

'Yellow foamy stuff' on your oil filler cap can be an indication that there is a contaminant in the oil. Often, it is water from a leaking cylinder head. I doubt that this is the case this time though. It's probably a reaction between the two oil types.
The yellow paste is an emulsion. I would guess that a change of oil and filter should do the trick. Get it done sooner rather than later, as if too much of the oil emulsifies, you could end up with clogged oil routes and excess wear or overheating. Don't use oil additives either, most people reckon they're crap.
Let us know what you do to solve it.

[AudiSport]

I had this same problem and I'm afraid to say it turned out my Cylinder Head had cracked. The car had only done 70k too.

My fingers are crossed for you, but do get it checked ASAP!

[FroggleRock]

Thank you both for responding.
After being told over the phone by 2 technical services (one Audi, one VW) that it's only condensation, I just decided to wipe the residue off and do some more miles... I'll obviously keep a close eye on that and let you know what happens.
Yes, finger crossed.

[Marc97]

I was gonna suggest condensation due to the very cold weather.

The engine getting hot then cooling down again can turn even the slightest amount of moisture into steam which ends up on the oil filler cap.

Hopefully not a head/gasket, fingers crossed.

[onzarob]

My Hillman imp used to do this all the time, at best its condensation at worst its a head gasket. You don't say how far your girlfriend drives each day. If it only a few miles (less than 10) then the engine will never get completely warm through expelling the condensation. your girlfriend will also drive a little easier than you not warming it up as much.

A guy at work at an Octavia 2.0 petrol and it does the same. 6 miles each way to work.

Rob

[skymaster]

My Mums A3 is the same, iT'S A 1.6 on a V reg with about 40k on the clock. I cant see any oil in the coolant. I might get it's oil changed and filter done for her and use some decent longlife oil this time.