I have used Royal Purple for the past 7 years in my 2004 GMC Envoy and 2003 Dodge Ram 2500. I have it in the engine, transmission and rear end and run 5-7 thousand miles between oil changes. My fuel milage in both vehicles went up from 2 - 3 miles per gallon in both vehicles. That is a conservitive number. I also put an over size tranny pan and rear end cover on both vehicles to help with cooling in the desert heat. I love this oil. I also tried Lucas but it did not give me the same fuel milage gain.