waste of money imo. just get 87, i mean like soeone mentioned earlier, is 2-3 hp, or 1-2 mpg really worth that extra 20 cents per gallon, or about 2-3 dollars a tank. that's about an extra 2 gallons, or 60 miles. versus about 15-25 at the very most exra mpg by using premium. u decide. some ppl just feel safer on the 91, and i don't blame them.
btw, doesn't the 330ci require 91?