What is the best all around fuel additive? Which ones have you tried, and why do you like them? Personally I have used the Motorcraft Cetane additive, Howes, and Opti-Lube XPD (yes, I saw it in a magazine and I gave it a shot). As far as MPG, I didnt see any real improvement w/ any of these, but with the ULSD I do believe that you should run some sort of extra lube. All recomendations will be greatly appreciated.