Q: steps to replace head gasket on 2001 Chevrolet Monte Carlo

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oil seen on radiator cap
(2) Answers
oil in the radiator usually indicates a cracked block. coolant in the oil is an indication of a head gasket.
also, are you sure it is oil and not trans fluid?? if your trans cooler leaks, then the radiator has to be repalced and both the cooling system and trans has to be flushed.
have it checked to confirm before you do this repair.

Looks like a bad oil cooler. If the motor oil in the engine is clean and not milky, the cyl. head gasket might be good. Since the oil pressure is a lot higher than coolant pressure, the oil will be pushed into the radiator through the failed oil cooler.
Get the cooler replaced and the radiator flushed out from all the oil.