Overheated, changed thermostant and filled w.

Most likely a leaking head gasket. Get that coolant out of the oil right away. It will melt the soft first layer of the engine bearings, then you'll have a bigger problem.

Depending on which engine you have, the first place to look on the way to the head gasket(s) is the intake manifold gaskets. To help with this, add a small bottle of dye to the coolant, then drive long enough for some coolant to be lost. Drain the coolant, then when you remove the intake manifold, use a black light to search on the gaskets. There's coolant passages in some intake manifolds, and those passages go into the heads, so a leak in the manifold gasket can let coolant get into the oil. The dye will show up as a bright yellow stain. You'll see that inside the passages, but you're looking for it in cracks in the gasket.

Intake manifold gaskets leak a real lot less than head gaskets but you never know; you might get lucky and not have to keep on going all the way to the head gaskets.

I find it hard to tell because there is going to be some coolant dribbling down from the passages just from taking the engine apart. If the intake manifold gaskets are made of thin steel, look for a section that's eaten away between the coolant passage and areas where oil can be. The oil normally runs around and onto the tops of the lifters.

There won't be coolant passages in the intake manifold for four-cylinder engines. With those, the coolant has to get into the oil through a corroded spot in the head gasket between the coolant passage and oil drain back passage.

I'm not good at locating leaks in head gaskets. To me it's like watching a doctor read an x-ray. He points, but I still don't see anything.