"This scab hasn't been pic
"This scab hasn't been picked in a while!
I use SAE 40... engine oil. If you use the boat in primarily the warmer months or are in a fairly constant temperature zone (like Florida) Multigrade oil (10W- or 20W-whatever) doesn't do much for you.
Synthetic oil isn't any slipperier than petroleum based oil, but sure does cost more. Folks will argue that synthetic oil will make the engine last forever, where other oil will let you down.
All other things constant, (temperature, type of use, etc.) the most damaging effect on lubrication comes from contaminants in the oil. Synthetic oil is not exempt. Engine wear products, water, and combustion products build up. Under normal conditions, oil doesn't wear out. But, the oil filter only gets rid of the big chunks. For the small engines we are discussing here, there isn't any type of purification system, only replacement.
Regardless the type of oil, if you don't routinely change the oil and get rid of those contaminants, the life of the engine will suffer.
Perosnaly, I don't see the point in spending 3-4 times the money for synthetic oil."