Yanmar recommends 30w for the
3gm30. 30w, 40w, and 50w all are very close in viscosity at operating temperature after the
engine is warmed. They are not close in viscosity at cold temperatures such as 70-80F. The only problem with using the 50w
oil will be not getting quite enough lubrication during the warm-up period for the engine. After warm-up, the lubrication should be OK. I assume you are talking about 50w and not 10w-50, which is quite different. Considering the
price of oil versus the
price of engine rebuilds, if it were my engine, I would buy some 30w and perhaps use the 50w for topping up the
motor oil when needed.