Coolant mixing with oil

Sounds like a possible blown head gasket. This is the gasket that is between the top and bottom halves of the engine. It is importance cannot be overstated. The top and bottom halves of the engine both expand as they get hotter. When it gets overheated, the halves of the engine can expand too much, causing that gasket to be ripped. Often times, when that happens, coolant is allowed to mix with the motor oil (the gasket separates them).

Here is an article that goes deeper into detail and lays out some diagnostics that about anyone can do: https://www.2carpros.com/articles/coolantantifreeze-in-the-engine-oil