I, too, use mostly Rotella 15W40. However, I also use a lot of Mobil Delvac 15W40, which is another excellent oil. In my opinion, you can't go wrong with using ANY name brand 15W40 oil...Valvoline, Shell (Rotella), Mobil (Delvac), Quaker State, etc. Maybe the type of oil you use might make a...