[blue2000s]

Because the air is less dense in the mountain states, the engine is less sensitive to knock, hence the lower octane requirement. 91 in the mountains is equivalent from an anti-knock standpoint to 93 at sea level. You WILL NOT make as much power as a car at sea level, but it's due to the density of air, not the octane of the fuel. Adding octane booster is a waste of money, and actually will drop power even further. In other words, what Kenny said.