FWIW
When I converted from Delo 400 to Amsoil, I used the Amsoil flush before I drained out the Delo. And it is all you need to do, according to Amsoil. The flush eliminates the need to drain out the expensive synthetic oil after only a few thousand miles, because it cleans out the gunk that the dino juice oils leave behind. And re: mileage, After I converted to Amsoil, and installed the Superchips tuner programming, on level 3, My highway mileage went from 18. 5 to 22. 7, verified by my Magellan GPS and hand held calculator, and checked against trip computer, which is amazingly accurate, BTW, at least in my '01 QC, w/3. 55:1 rear gear, 47 RE A/T,and 19. 5 Alcoa's on it. This is no towing, no tailgate, locked on 65 mph in cruise control mode, driving the spine of the Appalachains and the Alegheny mtn ranges, and some of the Pennsylvania Black Forest. In other words, not flat as a table, straight, level running. So, yeah, changing to Amsoil and the full flow/bypass system was the best thing I have done so far. I can't say exactly how much difference was the oil, and how much was the tuner, but I'm pleased with both mods. YMMV