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sometimes that stop leak crap can look like oil floating in the resevoir. but before condemning the head gasket a cooling system pressure test should be performed.
its probably a head gasket. or your head is cracked
Have it pressure tested or you can do it yourself if you have a radiator pressure tester. They cost around 50.00-200.00 and you put it on your radiator (where your cap goes) then you pump it too pressurize the cooling system and you wont maintain pressure if you have a blown head gasket.