Thanks for the info Norrin. A mechanic at a repair shop in West Yellowstone told me something similar. He pointed out as you stated if you fill up at 6,500 ft. then head down to lower elevation the engine may develop spark knock when under load. So as the engine mfgrs say to run their product at minimum 87 octane I do not want to assume the risk of potentially damaging an engine to save .20 a gallon on gas. Running a vehicle with less than the mfgrs suggested octane rating could potentially void the warranty. Though I was surprised by the amount of people filling with the 85.