Just recently I notice a little oil in the antifreeze reservoir on my 96 Lumina. Could this be a sign of a problem? What do I do to fix it?

 

Are you sure it's oil?

These cars use crappy Dexcool, which deteriorates & leaves a nasty film & some floaty crap in the coolant reservoir.

I suggest you clean the reservoir (remove it & wash it out thoroughly) flush the cooling system & refill with fresh Dexcool.

Check the reservoir again in a few days & see if any new 'oil' appears. Let us know what you find.

 

I agree. Check the oil closely. If it's like a milkshake, that would be a problem (head gasket).