I run 88 non ethanol in over 20 machines every winter with no issues and no det. This is on everything from 550's to 800 pro's and etecs. We put over 2500 miles on each of these machines in a season.
FYI, if you do the math, 91 at sea level is 85.8 at 10k, so 88 should be good down to about 5k with 88.
People worry about octane so much, but at higher elevations it's not the same game.
Thanks for all the answers. My thought is to run the No Ethanol fuel as much as possible (its not always logistically feasible) and certainly the last couple of tanks before summer storage.
I've always gone out of my way to get non ethanol fuel. My etech doesn't seem to run as good with ethanol fuel might just be a coincidence that it didn't run as good but my brothers pro really seemed like it ran like garbage on it so we drive opposite direction to go get good fuel. Just my opinion though