it could be a blown head gasket causing mixing of oil/coolant resulting in the creamy mayonaise that you describe especially if there is a lot of it on the oil filler cap inside aspect.

however ,if it is only slight then it could be insignificant-it all depends what other symptoms you have such as loss of coolant,if similar mayonaise is noted on the oil dipstick-these would point to a blown./leaking headgasket.

get the cooling system pressure tested first to get the diagnosis.