Oil threads always have opinions. So here is mine. If you read the VP manuals for my 4.3L they have SAE30 for the engine oil for any reasonable ambient temp you'd want to leisure-boat in (3.0L is a relatively similar GM pushrod engine, just a single cylinder bank) which is quite viscous compared to the Mobil1 that I see in auto stores around me. 5W-30 seems to be about it...and I realize the idea is to have a 5W viscosity when cold and a 30W viscosity at high temp, but I'm pretty confident that a SAE30 is always going to be more viscous than a 5W30 at any temp...(hence the move from 10W30 to 5W30 as a fuel economy initiative...its less viscous hence less internal drag).
So why the extra viscosity? Boat fueling calibration runs rich in almost all conditions for reliable power, nice throttle response, and no catalytic converters. However the excess hydrocarbons running around also breeds fuel dilution of the engine oil....which is why I think the dino oil, changed regularly, to be a better solution. There is marine grade synthetic VP oil, my guess is its additive package improves viscosity while retaining a synthetic formulation.
I use synthetic oils for 3 reasons 1) engine is air-cooled, the only fluid cooling is thru oil, synthetics are better at extreme high temps. 2) oil system used as a hydraulic system for disabling valvetrain components in "active fuel management" which shuts off cylinders, see current GM small block V8. Synthetic is a more stable and reliable hydraulic fluid than the dino oil, less prone to aeration which causes weird stuff to happen in a hydraulic system. and 3) in my commuter car because I believe my personal choice of a 10k mile oil change interval saves money over changing dino oil at every 4k. I do change the oil filter at every 5k, to continue to remove contaminants.
My boat could get 50hrs of use next year. That's the equivalent of about 3500 highway miles in a car. I'm going to stick with the basic, dino, SAE30 for now. Your mileage or useage may vary.
