What gas octane???

At altitude, you can safely substitute 85 octane fuel if your engine is designed for 87 at sea level, unless of course you're running a supercharger, turbo charger, or have an exotic variable compression engine of some sort. Less dense air on input results in lower absolute pressure in cylinder and hence, lower octane requirement as you won't reach detonation pressure in a fixed compression cylinder chamber. Keep in mind the result most noticeable to the driver will be a loss of approximately 3 percent of power for every 1000 feet of elevation gain on a modern engine that compensates as much as possible to the change. Power loss on an older manual carburetor type engine is even more unless you get out periodically adjust the carburetor for optimal air fuel mix.