[divezic]

Also, the higher altitude you drive the lower the octane requirement. The gas stations in western Montana sell 89 octane as premium and I think 95 octane as the lowest grade.

My guess is that the Chevron fuel system cleaner claims to remove carbon deposits from the cylinder walls. These deposits cause "hot spots" which cause detonation or pinging. I don't think the cleaner actually raises the octane rating of the fuel in your tank.

If your car is detonating on a higher octane at 7000 feet, you probably have something wrong with your ignition that a higher octane won't fix.