I look at it this way, if Mobil 1 makes good oils (and they do), and if they claim that one of their oils can go 20,000 miles “guaranteed”, then that has to be their best oil…something they do to it (base stocks, additives, whatever), has to make it better than their other oils. Because to have an oil that can go 20,000 miles isn’t easy. Unless it’s all marketing, and they can market it that way without consequence, and maybe they can?? How can you prove it isn’t? It’s not like if one of their other oils could do the exact same thing you could sue them, right?
Having said that, I’d consider their 0W40 FS Euro to be their “best oil”. At least I know (or think I know) that it has around 40% PAO, and I definitely know it has MB, VW, Porsche, and BMW certifications/licenses. And those aren’t easy to acquire. I also assume it’s not an oil for everybody (direct injected turbos because of the calcium and LSPI, maybe).