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>