Look, if you are going to spring for synthetics, then go with the best. Get the 5-30 series 3000 heavy duty diesel oil, you will not be sorry and the truck will continue to run at it's best. I have run the Amsoil for the last 5-6 years, and before that I ran the Mobile one(back before they watered it down). Synthetics work, and work well. Go with the best, weather it be engine oil, transmission fluid, gear oil, or grease and don't worry. I don't think it makes much sense to run anything but the best their is.