Hi,
Don't get too stirred up about the mayonaise in oil filler cap thing, its very common on the V 6 engines.
Its a bit like hearing of someone having a serious illness, you can soon imagine you have the same with a few symptoms.
I know you may be able to get a garage to carry out sniffer test on header tank, but can you smell any petrol / cold morning rich exhaust sort of smell from the header tank ? If you can't then it's not likely head gasket.
As the oil pressure is higher than the pressure in the cooling system even when running, you will see definate oil sheen and yuk in the water if it is the oil cooler. If there is much water in the oil you will have white yuk on the oil dipstick.
Slow water leaks are a pain to find sometimes, when hot, the water evaporates before you can see it, there are plenty of possibilities on Omegas, The HBV, the heater matrix, water pump, all seem to be very common. If you can pressurise the system cold you may spot them easier.
If all you can really notice is a slight loss of coolant, look for some of the simple things to leak before assuming the worst.
Roger