About 3,000 miles ago I switched to AMSOIL series 2000 75-90 in my rear diff. Shortly after I began to get a grind from the rear while pulling a load. Around corners and such. It sounded like a rear diff that needed the friction modifier. I switched to Valvoline and the noise is 100% gone!!! Towing etc. I loved the AMSOIL, but dealer said that the friction modifier that is already in it probably cause the rear to slip, etc. It sure quieted down the howl, but just didn't work in my particular truck. Great fluid though!! 
