What was the OCI for those cabs, Dual? If they're doing 3K, 5K OCI, it wouldn't matter if they ran M1 syn, or any SM-graded Dino out there, or SL either for that matter. This inference that M1 is the reason those engines are clean is SUCH a lie. We're used to that from Exxon/Mobil of course, which is why it's fallen out of favor a bit around here. Also, there are fewer lemmings at the AA and AZ buying the stuff, too. Every time I'm in there, WalMart, too, the bottles are sitting there, fully stocked, dusted over with age, and all the other brands moving.
The OCI is the reason those engines are clean, and it doesn't matter much what the oil is, especially at the OCI I can imagine they used, 3K or 5K tops. Heck, standard-issue M1 is a GroupIII conventional oil anymore, there's certainly nothing special about the stuff anymore, and the price only goes up on it.