I went through the same thing when we bought our 2010 Cummins. Owners manual calls for 5W-40 full synthetic or "engine damage may occur". OK, I call up the dealership and ask them about this. I tell them that the other Cummins we had never had this requirement (1998, 2001, 2005) and all I ever used was 15W-40 conventional. They tell me that they use 15W-40 conventional in all their Cummins and have not had any problems. So I ask if I use 15W-40 conventional and have an engine problem, will they warranty it? Crickets at first and then the music began with the dancing to follow. So I've been using the Valvoline 5W-40 full synthetic. Cheap insurance.