@JimP this would be a corner case situation you should be aware of. I do know 4 other people who ride Grizzly ATVs this has also happened to. Yes, most people on here will never need to be concerned with or need to deal with this. It is highly suspected the contributing factors are ethanol pump fuel, warm temperatures and high altitude.
I actually encountered a situation that spooked me a bit. Mid summer and warm out, low 80s at about 10,500 feet and my Grizzly completely stalled where it would not restart or when it did, it sputtered out and died. Fortunately I was only on a dirt road and not in an extremely steep section of trail like I was about 1 hour earlier. Sat there for a good 10 minutes trying to get her started and my riding buddy came back to find me. He said he knew what it was as he had this happen before, got his fuel container off his Grizzly and poured about a gallon of fuel in the Grizzly. Added to probably about 1 1/2 gallons 10% ethanol fuel in the tank. My Grizzly started right up like nothing was wrong. Bill mentioned the cooling of the fuel is really what helped but that he only ran non-ethanol fuel in the summer because of most likely micro-bubbles at the fuel pickup was messing with the fuel pressure. 10% ethanol fuel is supposed to have a lower boiling point at increased altitudes when compared to pure fuel.
Anyway, I ended up running pure fuel for the rest of the summer. The bad part is I typically was in areas where a minimum of 91 octane was available so that is what I ran. I never had my Grizzly stall again during the summer running pure fuel and yes, many trail rides to places above 11,000 and 12,000 feet.
Unrelated to higher altitude information, I'm not a big believer in running octane fuel higher than what is suggested by the manufacturer and I also am familiar with the following link that explains why higher octane than suggested can actually slightly reduce your optimal power.
SuperATV - AN OCTANE RUNDOWN—WHAT YOU DON’T KNOW ABOUT GAS
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@CanadianKodiak700 mentioned, the manual does say fuel type unleaded at a research octane of 91. That is a different rating system used outside of north America. I understand it that research octane 91 is equivalent to US/Canada 87 octane pump fuel.