My relatively new Yanmar 4JH3E was unused for 3 years and has just been put back into service and has a problem: milky oil and overheating.
Before I started the engine the first time, I changed the raw-water impeller, the radiator cap, the thermostat and the pump/alternator drive belt. All the old parts were in good shape upon removal. Basic maintenance is the limit of my knowledge and ability - but I've always been good about doing it on a timely basis.
After launch, the engine overheated. Part of the overheating was evacuation of coolant: about a quart (of a total capacity of about 7 quarts) got pushed out of the engine and then the overheating happens quickly.
There is liquid in the oil - very milky. I've changed it each time as I've re-tested the engine and it gets milky again. It takes 20-40 minutes before the engine overheats (the gauge spikes to max in just a few seconds). The engine is now very hard-starting: you turn the key and there is a long pause as the starter tries to turn the engine and takes a few seconds to overcome the engines resistance to wanting to turn.