Just bought an 08 GMC Sierra Dually 4x4 SLT Duramax w/6spd auto tranny. Now my head is spinning with all this stuff im reading LOL. First off what type of motor do i have LLY, LBZ etc. I pull a 30ft toyhauler and i am wanting to improve mileage like everyone else. I have plenty of power for my need just want to improve mileage without doing anything too crazy or risk hurting anything. I am going to put the amsoil by-pass kit on after break-in. What about fuel additives? Do i need them? I live in Ohio so it can get a little cold in the winter. Do they help mileage? From what i understand, i can use any biodiesel fuel. I was also thinking about a programmer after break-in. Will this void my warranty and will they know if i have one installed if i have it serviced. I have more questions, but i wont kill everyone yet LOL. Thanks for any help i can get.