Re: Confused about which oil weight to use
DixeyJulie, to have an engine seize through overheating once is bad, twice is terrible!
You say you have water flow through the exhaust, so the raw water side seems OK. However, what is the thermostat like? You could remove it and try running without it. However, if your engine has a sealed (freshwater) system which is being cooled via a heat exchanger, there could be a problem in that side of the system. Low coolant level, an air lock, circulating pump shot. An air lock? Sometimes happens after the coolant has been changed.
I doubt the wrong grade of oil could cause overheating like you describe. If there was a lack of oil pressure due to a blockage, failed lube pump, low oil level or foaming you should get an alarm or red light warning in time to shut down.
When you do shut down make sure you let it idle for a few minutes first. Although Yanmar recommend running the engine up to high RPM if it has not been working very hard, they still expect that after having done that, the operator will bring it back to idle for a few minutes to stabilise temperatures.