I've been toying with the idea of running it in my and the wife's Yamaha Grizzly 660 but haven't done it yet. I changed it with Rotella T6 and Fram Ultra last and haven't put enough hours on it yet to justify changing it! Lol
But honestly, I'd have no qualms about using it as it shows great results in PCMO and HDEO applications here. The problem with my situation is there's no Walmart nearby within 100 miles so I can only get it if I'm on a trip out of town and happen to come by a WM.
And modern ATV engines are more efficient and tighter tolerance than ever. They're quite easy on oil as long as you keep your air filter clean which is even more important here than anything else as they're obviously exposed to way more dust/mud than cars by a long shot! I use M1 in my 400EX because it's available here and does well in the extreme heat air cooled engines build up, especially in the dunes on a hot summer day climbing a 100' wall of sand with heavy paddle tires and a 200lb rider aboard. But I believe the ST would do well in it also, it's just the M1 I can get right here locally and can't with ST.