Greetings to all, A newbie here from CT. and thank you all for any info you may have to my future ques, To me the dumbest question is the question NOT asked. I'll have a few of them as time goes on. I just bought a 06 GMC sierra 350, 130k, Allison 1000 6speed, 2nd owner, dually to pull a 5th wheel trailer. What does LLY and LBZ mean? I have a LBZ motor. I'm am almost done in changing all the fluids, not knowing when this was last done, I would do it now and know. I am still on the 1st tank of fuel and have noticed what I think to be a low fuel mileage avg. at 12 mpg, I haven't pulled the trailer yet and most of the mileage so far have been highway. Does that seem low or is that avg for this motor? If low, how can I improve it? filter was just changed as well.