There are differences in brands of 15/40 and most any weight oils . If you really dig into engine manufactures guidelines for that specific engine the oils have different additives and when you look on your oil you are using on the jug will be a circle and 2 letter ratings like sp ,sn, sm and on and on GM /FORD/whoever .All seem to have different things they want to see in their engines and a big difference is turbo power engines some oils foam real bad in the turbos when they get hot and wind up wrecking bearings inside so if your engine has a turbo pay attention what you put in it . Back in 95 i bought a new powerstroke in a ford truck and at the time i didnt know about the specs and was using dello 15/40 and it didnt have the 2 letter ingrediants but Rotela 15/40 did and Ford dealer said if i cooked a turbo warranty wouldnt cover it