I am a HUGE fan of redline products, but with the frequency of breakdowns with my 2004 6.0 (2003 MY engine), I have decided it no longer deserves the benefit of an oil that expensive. FWIW, cold starts are smoother with delvac 1, and surprisingly, even more so with rotella syn. In my case, I believe the HPOP and injector components are getting "tired", hence the improved cold start behavior with 5w40, along with better turbo response. Redline 5w40 wasn't listed as a diesel app until recently, so I haven't tried it, but I'm sure it would be fine. I won't spend the $$$ anymore, as my truck is almost paid off and I intend to look at the new cummins plants if chrysler survives. My 6.0 no longer has a single original injector, all replaced under warranty. I'm looking for a smaller diesel, but the proposed 4.4L f150 is out. I refuse to buy an engine designed with timing belts.