Ihave a 2004 Mh with a 8.1 chevy engine. My Question is the manual says 87 octane. I live in Montana@ 5000 ft and they have 85.5 octane as regular gas. Since my manual calls for 87 as a minimum should I use a higher grade or is the 85.5 OK at this altitude? I know that the computer will retard the ignition to accommodate the lower gas but would I get better performance and mileage if I use the 88.5 gas? Any Info Would be appreciated. MB.