MObile one 15/50 has a flash point of 230 degrees.
Mobil one 20/50 has a flash point of 270 degrees . I am going to try the 20 / 50 when the motors go back in this week . 20/ 50 is scooter oil , but the stats are impressive .

Talk to Hydrocruiser, he is da oil man!

what kind of motors are they?

I have been changing the type of oil that I use all year long. I have oil temp issues and have been trying to correct it all summer. I used the M1 15/50 and after about 5 hrs, it was black and the motor was starting to use oil alot. Switched to Mobil 20/50 and it doesnt burn any and the oil is just as clean as when I put it in. Temps are still around 280 at WOT, but the 20/50 seems to be better for my app.

Just a correction and this is a pet peeve as a ExxonMobil distributor.. Mobil 1 is spelled MOBIL 1, not MOBILE

PS the 20w50 works great so does the 15w50. I sell a hell of a lot of both to boaters.

Mobil-1 V-twin FP is 518*F and 15W-50 is not far behind.

If your boat "lives at Wide Open Throttle" literally constantly...go V-twin for sure...otherwise the 15W-50 is fine.