85 octane

wolfe10 wrote:
As stated, when at elevation, 85 octane is just fine.

BUT (read that a large BUT) this means only when driving at high elevation.

So, if you will be in mountain driving at lower elevation (say, when you are returning from high elevation to lower but still in some "upgrades to pull", do not fill with the lower octane.

We face this when we travel from home(elevation 22') to Angel Fire, NM (elevation 8,400'). No problem with 85 octane while in the Angel Fire area, but we wait until lower elevation to fill up when returning to Texas. The other option would be to fill with mid-grade when at high elevation stations.

This might cause a problem in an older engine but the new computer controlled engines, which have been around for a decade or more, just adjust things so you have no problems with your engine. I am now approaching 90,000 miles with my 8.1L GM engine in our motor home and I always use 85 when it is available. I have never had a problem if I still have some in my tank when I get down to sea level and I monitor my engine operation with an OBDII gauge at all times. The computer adjusts for the different octane, just like it adjusts for the varying amounts of ethanol in my fuel. All of the ECM controlled engines have knock sensors that will adjust the timing to prevent any damage from a long pull with a lower octane fuel. You may notice that you have a little(very small) less power from 85 over 87, but I doubt if you will actually feel the difference. The difference in mileage is impossible to gauge in the mountains.
If you are worried about the knock sensor not making the necessary adjustments to your ECM when you use 85, than your engine will not make the right adjustments with 87 octane either.
If you are concerned, by all means pay the extra and get the fuel that you feel is appropriate.