Well, the MK2 is playing up!!!
I suspect it could be the head gasket but I’m not too sure???

Reasons to think this:
- Losing coolant but there are no apparent leaks.
- “Mayonnaise” on the oil cap
- Coolant looks slightly brown

Reasons against:
- Temperature remains normal.
- The coolant does not bubble
- No smoke from the exhaust
- Head was rebuilt only 8k miles ago

I’m going to try changing the oil filter housing and flushing the system first but does anybody have any ideas as to what else it could be?
What else could cause water to mix with the oil?
I really, really can’t be bothered to replace another head gasket!

Cheers,
Gary.

test the coolant for burnt fuel, cant think of the correct test name, but the white goo on the filler cap isn't looking good,

Oil cooler could be split and leaking water into the oil, but normally when these go they make a mess and fill up the oil with water.

Head may be warped, did you get the head skimmed?

Check behind you dash for signs of leaks from the heater matrix.

Make sure coolant cap is tight, if it is loose then it might be evaporating, I have seen this before.

Chris