I see the shelves where I have been buying my Rotella 15W-40 is now inundated with Rotella Synthetic 5W-40. I like the idea of synthetic oil but I question the wisdom of putting that weight oil in my hard working klinker. The back of the container says that it is approved for Ford, Dodge and GM diesels, but it also says to check the owner's manual, which clearly states 15W-40. My truck never sees sub freezing temperatures, but I'm finding this oil on the shelves here in Mesa, Arizona where it never freezes. Should I use it?