I use 15 - 40 W Amsoil Diesel Engine Oil. 20 quarts is what mine calls for. However, I always run mine just shy of a quart low. I get almost no blow by and I don't worry about having foam in the oil pan. Also, may I recommend the Fumoto Engine Oil Drain Valve
If you are a slob like I am, you will love this valve. Great for not only changing oil but each year I do the S.O.S. oil analysis. Just slowly turn the valve, fill the sample bottle, then shut the valve. NO MORE MESS.
1997 Triple E Empress, Freightliner Chassis, CAT 3126 w/ 275 horse's.
MD 3600 Tranny, 2011 Ford Escape Hybrid w/Blue Ox tow bar and Roadmaster Base Plate.
Amsoil Synthetic Oil only for my baby.