I recently moved from the SF bay area to near Prescott, AZ. My Evo VIII was tuned by WORKS for 91 octane. The elevation here is 5000 ft. Can I safely use 89 octane? The premium here is 93 and about 25c more per gallon which is $3.89. I've used the 89 and didn't detect any drop in performance. BTW, the engine was recently rebuilt and has 2300 miles on it. I also have used a fuel injection cleaner additive. The car really likes the cold mid 20's mountain air. It'll break all 4 tire loose!