1999 Volkswagen Golf yellow cream under oil filler cap

As you say , if no coolant loss and no overheating , probably just condensation .Short journeys in this weather when the engine never really gets to working temp for very long , doesn't help. Could check engine breathing / hoses are clear .Noticed my daughters car was suffering from same problem last week and had visions of head gasket also , but seems o.k .This cold weather doesn't help .

we had a cure for this problem in british telecom when i worked there.
it sounds crude but it works.
the sludge is formed during short runs.
we placed a piece of thin laminate in front of thr rocker box secured by what ever works on your car,
its purpose is to deflect the cold air over the cool metal cover.
you could even try a thermal cover like a hot wat cylinder cover.
all it has to do to allow the engine to heat up
also see if the cooling fan is constantly running
as this will cause engine to run cool

thanks for your info,the fan is working fine