Delo may in fact be marginally better oil than Rotella-T, and I've used it many times, but Rotella-T hasn't hurt my old 6.9L much. If I could find Delo easily around here, I'd probably use it. When I find Delo, it's always substantially more money than Rotella-T and I don't think the difference in quality is as substantial as the differnce in price. When I got my Ford 19 years ago (my first diesel- still have it), I wondered which oil to use. When on a trip across country (CO to FL) towing a 30 foot travel trailer I took note of something. Back in those days, you had to use truck stops more to find diesel fuel. I started looking in the trash cans to see what oil the truckers were using. At that time, Rotella-T was everpresent and that became my oil of choice. Scientific, huh?