Overheating

Actually, these tests are not going to be of much value. Compression readings are typically normal when the head gasket is leaking slowly into the cooling system. A better test would be a cylinder leakage test. That involves pumping compressed air into the cylinder, through the tester, and observing the percent of leakage. The big advantage is you can listen and look four places to determine the cause of that leakage. If a head gasket is leaking, you will see a steady stream of tiny air bubbles in the radiator.

Leakage of coolant into the engine oil is by far the least common head gasket failure. Both systems are under similar pressures when the engine is running, and they're both real low. The most common cause of the white, milky-looking gunk under the "oil"cap is condensation from excessive short-trip driving that does not give the engine a chance to reach proper operating temperature. You will even see water droplets that you can knock off, and they will be clear, not green.

The engine size wasn't listed, so we do not know if a leaking cylinder head gasket is a common suspect, but that will cause overheating. The best test is a chemical test performed at the radiator, with the engine running. That involves drawing air from the radiator through a glass cylinder with two chambers partially-filled with a special dark blue liquid. If combustion gases are present, the liquid will turn bright yellow. You can also add a small bottle of dark purple dye to the coolant, then check a day or two later with a black light. The dye will show up as a bright yellow stain that you can follow back to the source. If the head gasket is leaking, you will find that dye inside the tail pipe. Auto parts stores have the dye for the fluid being tested, and those that rent or borrow tools should have a black light.