If you are talking about the yellowish 'mayonnaise' thats present under the oil filler cap, it means one of two things....1) The car is used for short runs and the condensation can't boil off as the engine doesn't get hot enough 2) The cylinder head gasket is leaking. I'd go for (1) so long as you aren't losing any coolant and there isn't any traces of oil in the coolant filling bottle. Don't worry, wipe the gunge away and take it from there.
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