I aquired a 2001 GMC Sierra 3500 dually about one year ago with only 57K miles. This truck is used in the non-winter months hauling a 3 horse gooseneck horse trailer around or our old Avion travel trailer - but really nothing more than 7-8K lbs. As soon as I got it I changed the oil, oil filter, tranny fluid & filter and both diffs and intalled new plugs (0.45 gap) and wires and had new manifolds put on both sides. It probably came with 5w30 dino. I first put in Mobil 1 0w30 - it burned about a quart per 1K miles. Next I tried M1 5W30 same thing 1 quart per ~ 1 K miles. About 3-5 K OCIs. Right now it has Delo 10w30 hdeo and consumption is 1 qt per 2 K miles which is not too bad in my mind for a BBC. I have been using AC 454 filters. I have read just about all I can online about these 8.1l 496 Vortecs and this seems to be a common issue. Some have apparently tried a different tstat and that has helped. Anyway, I think now that I have tried a 10w30 hdeo I will try Rotella 5w40 or 0w40 this spring as I have several diesel tractors and a 2003 Dodge 5.9 Cummins 1 ton and see if it improves more with a 40 wt. Some engine builers who specialize in the 8.1 (Raylar) even suggest a 15w40 hdeo and I plan to try that after the 5w40. I think in marine applications people commonly run these 8.1s on 15w40 hdeo and even straight 30 and 40 wt. I am also considering M1 15w50 or one of the 5w50s as well if I can find one. Any thoughts from those with experiences with the mighty 8.1?