I have a 93 Jeep Grand Cherokee with the 5.2 motor and it is rated for 87 octane fuel.a friend of mine told me that the bigger the motor the higher the rating and fuel it is supposed to use and recommended using 89.I know its true for sports cars and such they would have to use 89/91 or higher but would using 89 on a regular car like mine while its rated at 87 damage the engine at all?