All i use in the marine industry for inboard engines is 25w-40. Mercury Marine makes this (its manufactured by citgo) it is formulated for long use at high rpms. Like i said is thats what is used in all mercury inboard engines. The reason why the factory dosent advocate using automotive oils is that the 25-40 has extra friction modificators in it to keep deposits from forming and to reduce wear. lol as part of my certification they make us take a 5 day long course on lubrication and oils!

 

I'm a bit concerned. 20W-50 is not a common grade of synthetic nor diesel-rated oil. What brand did you find?

Your engine requires API CI-4 as a minimum. The Powerstroke injection system is unusual as it uses oil pressure to fire the injectors. It is vital to use the correct oil. 5W-40 Rotella T Syn is the most commonly available syn that meets the spec.

Jim