My 1998 Z71 half ton K1500 chevy with the 5.7 V8 just rolled over 159000 miles. Still great mpg of 20 on cruises and a low of 10 pulling my tractor. 16 most of the time on my mixed short trip driving.
It doesn't burn any oil and I have used Mobile 1 blue cap synthetic 5-30 since about 40,000. I had the intake manifold gasket replaced at about 30,000 miles because I noticed it consuming coolant. Since then, no IM issues and that is WITH the synthetic oil.
My 98 has the same 4l60e transmission and smallish 10-bolt rearend as the 2004 and the trans on mine is original and works great since being serviced every 50,000 and monitored with a trans temp gauge. The rear end needed to be rebuilt (bearings only, pinion bearing failed) at 150,000 miles despite mobile 1 synthetic for almost all of the 150,000 miles. Heavy towing at the GCWR is not good for this rear end.
Great truck. You'll love your 2004.
I also have a 2001 GMC Yukon with teh 5.3 and it gets even better mpg than my old 5.7. The steering shaft clunks but who cares? at just over 100,000 miles the clunking steering shaft is only a very minor noise when going over small bumps. It is not constant or irritating and if all they are going to do is grease it for a temporary fix then why bother?
Oh and on changing the transfer case oil. I did it and recommend it. Be warned that when you remove that drain plug oil will squirt several feet horizontally and could miss your pan. Pretty messy.
