To simplify, octane is an indication of how fast the gas burns.  The higher
the octane, the slower it burns.  If it burns (ignites) too fast, the car
pings or knocks.  Too slow, and you lose power, the same as if you retarded
the timing.  At higher elevations, gas burns a bit slower because of the
lower amount of oxygen in the air.  (Did you ever try to use a charcoal
grill at 8500 feet?)  Because of this, the lower octane burns at the same
rate up high as a higher octane would at a lower elevation.

What does this mean?  Higher octane is not "better" gas.  It doesn't give
you any more power.  It just burns a little slower.  If your engine isn't
knocking or pinging, you would get zero benefit from switching to a higher
octane.

Bill
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Remember, it ain't what you don't know that hurts you,
        it's what you do know that's wrong.