Why do people run multi grade oil in boat applications???

Biggest reason is probably that many of us run synthetics. To my knowledge, only Royal Purple offers a straight-weight DETERGENT synthetic oil. (Redline has a non-detergent straight-weight that is strictly for racing.)

Mobil 1 15w-50 Whipple Charged 502

Merc HP Drive Oil

Mobil 1 15-50 procharged 502s

Merc lube in the drives. Drained the lube this winter and found oil to be clean, except for powdery residue, and the same on the magnets. I will stick with Merc stuff.

I have Valvoline racing 40 in it now.

Come this weekend I'll be putting Mobile 1 15w-50 synthetic. Debated this for a long time but after reading on here about the results w/synthetics, decided to go with it.

Merc HP lube in the drives as well.