I have a Malibu tow boat with 340 HP 350. Indmar specs. Pennzoil 15-40 marine only but states Synthetic oil that meets our requirement of 15W40and API rating of SJ/CJ4 is acceptable to use after 100 hours or operation.(Still with 50 hour change requirement) I wouldn't worry too much about this accept I had a propeller rep tell me I could gain 150 RPM at WOT with a good synthetic. I pull a barefooter that takes the full rated engine rpm at WOT and am looking to prop up a little. Read some of the other posts and am concerned about auto synthetics in my boat engine. I would generally be looking to Mobil 1 which we use at the Chev dealership in all HP applications. what do you think?