It's a 2005 4.8is with 200k miles, and the engine is in great shape. We've had it just long enough to fill it up once, and I used 93. I wouldn't think of using 87, but 91 is a confusing number since it really isn't an option (87, 89 or 93 is all you see) Other than my motorcycles that always got 93, my domestic cars and trucks got 87 since they will run on any sludge you put in them.