Owners manual calls for 15w40, above zero. Synthetic 5w40 below zero. Im running CJ-4 rotella 5w40 year round....do you guys think im throwing my money away in the summer time sticking with the 5w40? Do you think the 5w40 synthetic will shear any compared to the rotella 15w40 dino? I can get a pretty good deal on a 10 quart jug of rotella 15w40, but have been using the 5w40. Chime in all!