Originally Posted by CMcAllister
Originally Posted by 6PakBee
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.


If you trust how the viscosity numbers work, and you can look it up on data sheets, 5w40 and 15w40 will be essentially the same viscosity once it gets up to temperature (210*F)




The key here is viscosity at operating temp. There may be a reason the need the oil to flow more freely on a cold engine.