Background: Friend'* 1999.5 Mark IV Golf 4-door with the 2.0L (auto).
Him and I checked his oil today and discovered he was a tad low and in need of a change soontime, as the oil was a little dark for our likings. I had about half a litre of 5w30 dino juice sitting in my car, so I let him have it.
Open the oil cap, and discover:
There was about as much sludge on the oil cap itself as you see in the pics, but I had wiped it off before I took the pic. I initially thought "oh ****, there goes the head gasket" - but YELLOW sludge? I'm accustomed to chocolate milkshake when coolant mixes with oil. Not banana milkshake.
Coolant levels (orange coolant) are OK. Engine runs smooth (as smooth as the naturally rough 2.0 can get). No overheating problems as far as I can tell. 140,000km (87k mi) on the car.