I ran 88 ethanol free in both my 14 Pro and 13 XM last weekend. Both machines are stock, and they ran great on the 88. I ride 7000+ elevation. As octane requirements are tied to cylinder pressure, and
as you go up in elevation the atmospheric pressure decreases and so does your cylinder pressure (unless your running a turbo). So in theory at least, an engine that is made to run at 91 octane at sea level (which all stock motors are) should have no problem running on 88 at 6000+ elevation.