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.