Ah, which is why I asked. The manual I have shows straight 30 for hot weather, or 10W-40. Looked at the link, and it seems the stiffest offered in Annual Protection is 5W-30, which incidentally is the grade I'm using in our four vehicle engines. Toyota and Nissan calling for 5W-30, and one Ford that was 10W-30 and another 5W-20. Looking at 15W-50 for the boat once the MerCruiser warranty's out. Using that for the ONAN would leave me with only two oils to stock. MerC's oil is a 25W-40 Syn/Dino Blend. Go figure.
The guys on the boat forum often include this question about oil selection: How many cases of engine failure from oil breakdown have you heard of? Consistent answer is NONE. Only case I remember in automotive was the early versions of 10W-40 did in fact break down and trash a bunch of car engines. Warranties were voided if it was used.