Car has 82,000 miles. Runs good, sounds good, cooling system very stable around 85C, no spikes. I recently changed from green coolant because there was a shiny black floating residue in the coolant tank.

Its still there. Soooo- besides head gasket, where else could black oily residue be getting into my coolant? The radiator hoses arent hard nor is the oil level going down so you could notice. Coolant level stable.

I'll do a head gasket if I have to, but how can I be certain that's the problem? Could oil get in through a water pump bolt or t stat gasket?