I was told many years ago that you should use straight weight oil because
when using a multi weight (IE) 15-40 the average marine engine does not reach 180 degrees (not including closed systems) therefore you are using only the 15 weight portion of the oil, the 40 weight portion only becomes effective at or over 180 degrees,dont know if this is true but it seems to make sence.
I use castrol straight 50 weight racing oil in my boat. I also agree that
Shell Rotella is probably the best, I use it in my trucks,front end loaders,Ect.
I am also a believer in the lucas products, I had a 1975 Lyman and the rear main was leaking and the lucas formula definatly worked. And I am not a big
believer of fix it in a can. lol Good fishing to all. Tinkertoy (john)