I've always had BBC's and have run straight 40wt or 15-50 engine oil. Going to winterize my fathers open bow with a Volvo Penta 5.0. What's recommended to go in it, 10w-40, 10w-30?

Volvo uses 30w Synthetic in all there gas engines

OK. So, is Mobil 1 0W/30 near the same weight? Haven't been able to find a synthetic straight 30w.

Oil threads are always a ford vs Chevy deal. From what I know multi viscosity oils are less tolerant to shear break down , hence the reason VP recommends straight weight. I have a SBC Volvo Penta and typ run non-synth 30W, sometimes I use a rotella diesel 15-40.

+1! thats what we use.