I'm using the Mobil Truck & Suv 5w-40 on a Dodge V-8 cargo van...heavy loader. I changed at 1K miles and plan on running it for 5K. What is the best diesel rated oil, I still say it's the Amsoil S3K 5w-30...but would a 40wt oil be a better bet on a heavy loader or would the Amsoil 15w-40 give better protection. I still believe the Amsoil is a better oil than Mobil..