hi ever since i got my car, i had to top up the coolant every so often, maybe half the exp tank every fortnight. my mums cav is the same so i thought it may be normal. but i thought the cooling system is a sealed system and shouldnt lose water and i thought id ask u lot. the oil cap has a little bit of a sort of creamy colour only after a short run from cold but after warm, the oil cap just has black oil on so i thought to rule out head gasket, is this correct? what else may it be?