i have traveled back and forth between washington state and wisconsin.along the way a few of the states have an unleaded with an octane rating of 85.5.i have never used it but have always been curious if this is ok to run in a vehicle that calls for an octane rating of 87.
When you are at a higher elevation (mountains) gasoline engines generally require less octane. If you are going to be back at your normal altitude before your next fill up, I would favor the 87. An occasional 85.5 is not likely to harm anything.