Due to the higher altitude and less dense air entering the engine, you can theoretically use a lower octane. However, in practice that is not always the case and occasionally you will get premature detonation inside the cylinders (pinging). This is one instance where you should move up to a higher octane fuel - to get a faster burning of the gas to match the timing of the spark and do away with the pinging.

In other words, if you hear pinging when you go up a hill or are otherwise under load, try using a higher octane. If you don't get pinging, there is no reason to increase.

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"If you hit them hard enough, anyone will put the ball on the ground." - Bronco Mendenhall