I saw 245 degrees oil temperature when towing my 5th wheel on those long uphills in the Rockies this summer. Do you really want to run dino oil in that situation. I think not. It normally runs about 205 degrees on the flats.
Rotella makes a 15w40 full synthetic.
My 2013 Ford owner's manual diesel supplement states 5w40 is to be used for severe duty situations between temps of -20 and 100+ degrees. Since it's their engine and I figure they might know what's best for it, I run Ford full synthetic 5w40 oil.
Ford recommends 15w40 only down to 20 degrees for my engine.