Curt, I will tell you what I have done; I change all the fluids to AmsOil Synthetic (engine, ATF, transfer case, front and rear diff) also the air filter to AmsOil, I was doing 11.3 MPG before and now it gets 12.5 MPG, not a significant increase, but it is some gain there; the main reason I did it was for the protection that you get out of synthetics especially from AmsOil. 
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"2003 Sage green LUX, wrapped around brush guard with off road lamps, roof mounted SMA light bar, AirAid Jr. kit, Delphi Touchscreen Navigation Radio with a 6 CD changer and more! Also a member of the HOPE (Hummer Owners Prepared for Emergencies) pro
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