Originally Posted By: aquariuscsm
Royal Purple as long as it's the Synerlec formula is one of the best imo as well.
I didn't think the RP 0w-40 had the Synerlec, having been SM, unlike the normal API grades at the time with Synerlec, which were SL. I was under the impression that the RP 0w-40 didn't have the Synerlec because of the ACEA specifications it claimed, but I could be mistaken about the reasoning. If I recall, though, the 5w-40 had the Synerlec and still might.
But, to the original question, whichever oil is certified for your application and is simplest and cheapest for you to get will do the job. I have great confidence in XOM, RP, SOPUS, and Amsoil. They all make excellent products. The M1 0w-40, of course, has a pretty impressive list of specifications and certifications.