I switched my K2500 98 GMC 7.4 to Synthetics<br>last year. Castrol Synthetic Dexton III, and Quaker state Gear oil in the diffs, Gained about 1 MPH, Use 0-30 Castrol Syn oil in the engine in the winter Nov to Mar( or if an oil change is due before March I use my regular 5W30) In other words the last change in the fall is to 0W30. I do get 16 MPG (CDNGAL) aroung town and 20 MPG HYW (CNDGAL) with my 454 Vortec!<br>Bill<p><p>
if your truck has highher milage(near 100k) i wouldn`t suggest using synthetic. i have a few Newer GM trucks and i use Mobile 1 synthetic, and Castrol synthetic oil in the other motor. I, however, don`t use synthetic fluids in the trans and diffs.