Hey JerryBob,
Reside on the Front Range Denver Suburbs and have heard for years about the effect upon altitude and the octane rating. Ammolab made reference to this and said he has no problem with 85 octane at about 6,000 feet elevation.
"As engines take in less oxygen per cycle, they produce less compression than they would at sea level. Because of the changes in atmospheric pressure, 85 octane at high elevation is the performance equivalent of 87 octane at low elevation."
I have never in the 37 years I have lived here had any problems with 85 octane. I would give it a try, if no performance problems, pinging etc. you may be wasting money paying for higher octane at altitude.
Scott