Mayo on oil cap.

Condensation will only generally build up if you don't run the car for a few days.
Not familiar with these engines myself but quite what else it could be if its using no oil or water, I don't know.

I posted the same question about my Mk2 GTI 8v about 3-4 months ago and, as the lads on here said, it was condensation. It was worse in the cold weather but always vanished when I took it out to stretch it's legs. It vanished in the hot spell we had a couple of weeks back.

Also, I had to change the rocker cover gasket and the breather grill was completely jizzed up.

Isn't that the same engine as the early mk4 2.0 8v?

Anyway, it seems I don't need to worry about it too much.

Thanks for the advice.

Not really. Condensation in the sump comes from moisture in the blow-by gasses that escape past the pistons. Every litre of petrol that you burn produces about a litre of water. Most of this escapes through the exhaust (which is why you see clouds of 'steam' from tailpipes on cold mornings, when the moisture is condensed by the cold pipe) but some ends up in the sump.

If you're mostly doing short runs, then you'll get a build-up of water, which can form an emulsion that looks like mayo. This is because on short runs (even up to ~10 miles) the oil doesn't get hot enough to evaporate the water and remove it from the sump using the crankcase breather.

Check that the breather is ok, and take it for a good run (50-odd miles or more) on the motorway to get it hot through, and see how it is then. Chances are it'll vanish.

BTW, if you're only doing short runs more frequent oil changes are a good idea to remove the additional contaminants produced by the engine always running cool.

IME, when head gaskets go they tend to go between a waterway and a cylinder. This can cause petrol scum in the radiator, a pressurised cooling system, bubbles in the header tank, steam from the exhaust, etc. If it's really bad the amount of water getting into the cylinders - and leaking into the sump past the pistons - is so great that you end up with mayo on the dipstick too.

However, if it's not using any water then I really wouldn't worry about the head gasket.