Oil in coolant reservoir
Maybe an eight of an inch of oil floating on top!
Is your transmission oil cooler internal to your radiator? It could be ATF in the radiator fluid and not engine oil. If it is engine oil, you may have a head/intake gasket issue, which is allowing oil to escape into the water jacket. Oil in the radiator fluid will negatively affect engine cooling, obviously the more oil the worse the effect - not to mention you still have the problem of what is leaking...
check your oil and see if it looks like milky
Looking for a Used Cavalier in your area?
CarGurus has 1,302 nationwide Cavalier listings and the tools to find you a great deal.
Search Chevrolet Cavalier Questions
Are you a UK consumer? CarGurus now has a discussion forum in the UK.
Chevrolet Cavalier Experts