Condensation. Vapors condense in this cooler area. Some cars are just prone to it. You should check your pvc valve. Does the vehicle use a lot of oil? Also do you do a lot of short trips?? An oil analysis could be a good move.
It's mayonaise! That's what we used to call it when oil and water mix in the presence of an emulsifier. Any acid would do for the emulsifier, as well as some of the additives in the motor oil. If you make trips shorter than 20 minutes in length, have a bad thermostat that isn't allowing the engine to warm up fully, or have a head gasket leaking, you may have this problem. I always heard that early versions of M1 were not too good at keeping water in suspension, therefore allowing it to collect on inside engine surfaces, but I thought that problem was solved by the SS. I hope your problem is a cheap one to fix, the end result will be sludge in your engine if it continues.
Dr. T, Best advice I can give is to change your oil now. It may be OK, but it's winter and it may take some time to evaporate any accumulated water out of your present oil. TBN is probably low at this point anyway. Even if you put regular dino in it, it would be better than leaving the old stuff in there.
I too have noticed this beige mayonaise-looking build-up under the oil filler cap of recent...but, I've had a failed thermostat so I'm not completely surprised.
Will it go away now that the thermostat has been changed? Has the oil been damaged or it's addatives depleted?