Actually, you can use a lower octane at higher altitudes.

Think about it in terms of the pressure in the combustion chamber. At high altitudes, ambient pressure is lower, effectively lowering the engine's compression ratio. There is less air to compress.

One of the reasons they turbocharge high altitude aircraft is to put that air pressure back into the engine, so it can breathe at the high altitudes, otherwise it couldn't generate enough HP to pull itself.

Avgas is a different beast, formulated for specifically for the different demands of an aircraft engine, there are several reasons not to run it in your car or ATV engine, more info can be found by searching Dejanews if you are interested.

 

Thanks for all the info guys, it is much appreciated.