Don't tell anyone, but I use whatever brand of Cummins-approved oil that's on sale. If it's Cummins-approved, it's good enough for me. Just read the back of the bottle.
Edit - anyone can tell you their experience with a truck or two, over maybe a couple of hundred thousand miles, but if they're not a Cummins engineer or a motor oil engineer, it's still just a personal opinion based on a small (tiny!) sample size and minimal scientific evaluation, if any. Give me a scientific comparison of 100 trucks run on brand "A", 100 trucks on brand "B", etc, etc, run over 500000 miles per truck, with religious consistency, oil analysis every 1000 miles, and a teardown of every engine at the end of the test, and I'll be impressed.
Or just call me a skeptic...