It used to be that gunge round the oil filler meant coolant in the system and a probable new cylinder head gasket. Whether that applies the other way round ie oil in the coolant I don't know. I'm not a mechanic so best wait for someone else
In general gunk that looks like mayonnaise in the coolant or oil would likely be the result of mixing oil and coolant under temperature. Often the result of a head gasket failure, but can be other things.
I'd check the appearance of the oil, both via the dip stick and by looking under the filler cap.
Could it possibly be caused by mixing red and blue coolant? The different types don't mix well and can cause havoc.