I find it amusing that so many people know so little about octane. I think the average consensus is the higher the better. Why is that , because it cost more? Simply, octane is a retardant. The higher the octane, the harder it is to ignite. Believe it or not, a lower octane fuel is a hotter fuel. The purpose of it is to prevent pinging or pre-ignition. Don't get me wrong, pinging is very, very bad. What is happening is the compression is igniting the fuel before the piston is top dead center. That is an engine destroyer!!! On the other hand you want good clean combustion. You don't want a lot of extra additives, read alcohol, in your engine from a fuel with too much octane. Harleys are not airplane engines. They are both air cooled but airplane engines have much higher compression, thus the need for such high octane. There is a reason why you can't get the higher octane fuels in the mountains. The air pressure and oxygen content is lower so you need a hotter, read lower octane fuel. They are doing you a faver! Don't sweat it, ride your bike anywhere you want to go and buy the fuel that is available. Its just what you need. I'll close with a good rule of thumb. I got this from a gasoline engineer who worked for Harley. It works for Harleys, cars and even airplanes. Use the lowest octane you can burn without pinging. Thats usually going to be the middle grade for Evos and 93 for Twincams. Remember though that is for sea level. The rules change in the mountains.
I hope this helps.
Hoghead4ic