OK...Don't do what I did. This is a public service announcement to take care of your vehicle. I have an '02 Duramax. I thought it was running really strong (was still getting ~19 mpg). Probably will do better now.
Mistake #1 - You are supposed to change the little red filter on the Allison tranny with every oil change. Not every 35000 miles. Auto trans fluid...black.
Mistake #2 - Don't believe the little air filter life indicator under the hood. After 2 years, I finally decided to replace my OEM air filter, even though the little thing said it was still like new (bar never moved). Air filter...black. Hi bug count, plenty of sand, had to vacuum out the air box.
Mistake #3 - Change your fuel filter every 15000 miles, not every 45000 miles. Just finished that one, as well. and the fuel that came out was chunky from water drops and what appeared to be large pieces of rust. Fuel color from the filter...black.
I think my next goal in WAY over due maintenance is going to be the differential fluid. Predicted color...black. The fuel filter is a PITA...must come out the inside fender well...good luck getting all of the old rubber seals off. The one in the middle is evil.