I diagnosed it as a headgasket as it was pushing oil firstly into the overflow bottle, then slowly more and more into the reservoir. Also when I left the car sitting for a couple days, on start up it would throw a big puff of white smoke. Couple weeks before, the car did get 3/4 hot on the temp gauge as a water hose came off. So I pulled the motor, got the heads skimmed and crack tested, gave the block surface a clean up. And put back together myself. No other symptoms, no smoke, no overheating, just oil sludge showing up in the reservoir.