There is plenty of reading to do on boost vs gas you run vs the elevation you ride.
Above 10 000 you can run most any boost you think your motor will take as there is so little oxygen you will not be getting deto. but you sure could blow crank seals.
If you actually ride mountain say 5000 ft or higher you dont even need premium on a normal aspirated sled. So I would strongly expect 91 would be all you need above 5000 ft for a turbo. Again assuming you are not boosting the hell out of it.
It is actually just math based on oxygen percentage of the air for the elevation you are at adjusted for the boost you add.
My girl friends dad who used to be somebody new a guy that had a wife that told me all this while her husband was at work.