He showed me how much to overfill it by and explained that the max given was based on a level installation and since the boat installation had the engine at a tilt it was necassary to overfill that pan.
Don't take this as gospel. It depends on which end of the crankcase the dipstick is at.
On commercial installations, part of the sea trial procedure is to carefully measure everything and file marks in the dipsticks which come blank.
The critical thing is to avoid having the crank contact the surface of the oil. My engine is installed well over the Yanmar tilt specs and I have had some interesting adventures with oil foaming.
My experience with Yanmars makes me doubt that the runaway in this case is the result of too much oil. I would put my money on worn rings assuming we are getting an accurate report that the engine does not respond to closing the throttle.
A critical thing we don't know is if the breather hose leads directly into the intake. My 2QM20 has a nipple for the breather hose to connect to the low pressure point on the intake and it turns out that it needs that negative pressure on the crankcase to keep from blowing oil out the dipstick. I've blown a lot of oil through that hose without a runaway. I now have a liquid trap in the line, just in case.