Obviously oil in the coolant tank?

Leaking head gaskets don't have to result in overheating. That happens when combustion gases are forced into the cooling system and that air pools under the thermostat. Thermostats open in response to hot liquid, not hot air. With slow leaks, the air will just get forced into the reservoir so the thermostat will stay open and the engine may not overheat.

https://www.2carpros.com/articles/engine-oil-in-coolant-or-radiator-gm-30-liter-v6

There's also oil and coolant flowing through the head gaskets. All engines have pressurized oil passages in the heads where oil can be pushed into the cooling system, and they have oil drain back passages where coolant under pressure can be forced into the oil. GM has more trouble with leaking intake manifold gaskets. That repair isn't so expensive but any coolant that gets into the oil is cause for concern. Oil will rot the rubber heater and radiator hoses from the inside and it will melt the first layer of metal on the engine bearings. That will get real serious real fast.

Nope; just suggesting other possibilities. Most commonly overheating and coolant loss are due to head gasket issues, but not always. Most commonly oil and coolant mixing or either one leaking externally is due to intake manifold gaskets, but not always. For visible external leaks in which you're not sure of the source, you can add a small bottle of dark purple dye, then after driving long enough, you search with a black light. The dye will show up as a bright yellow stain that you can follow back to the source. If you add the dye to the coolant and it shows up on the oil dipstick, you still don't know for sure where the leak is so the mechanic has to start by removing the intake manifold and hoping to find traces of dye between the oil and coolant passages. If he doesn't find that, then he has to move on to the head gaskets. Makes it real hard for him to provide an accurate estimate of repair costs.

Your welcome