I had a 2000 Cadillac Deville that "reecommended" 91 octane fuel. It started life as an Alamo rental car, so it had 2 chances of ever having been NEAR the high test fuel pump within the first 20,000 miles before I bought it: 1) fat and 2) slim! Consequently I ran it for the next 100,000 miles on 87 octane, topping it up with premium about once every 20,000 miles or so, out of pity for the pistons and a sense of guilt. I finally filled it from a near empty tank twice in a row with 91 octane gas at about 117,000 miles and it proceeded to blow a headgasket shortly thereafter. Coincidence? I don\'t know, but just maybe all that attention and octane were just too much for it to handle! BTW, I followed all the manufacturers service recommendations faithfully, including oil changes every 3000 miles, right up to the day I traded it for a new car. And all the work was done at the Cadillac dealership where I bought it.