oil in radiator

Have an 02 impala with coolant light on, rad and overflow levels are both okay, car runs okay also. I was told by a mechanic that radiator had oil in it.
Does that mean intake gaskets are bad or somethng worse?
Thought about cleaning or replacing coolant sensor.
I have had to add antifreeze periodically but not alot and figured if it wasn't broke, don't fix it. How can I be sure of the oil in radiator?