All GM X-Body cars must use unleaded fuel. The use of leaded fuel will plug the catalyst rendering it inoperative, and will increase the exhaust back pressure to the point where engine output will be severely reduced. The minimum octane for both the four cylinder and V6 engines is 91 RON. All unleaded fuels sold in the U.S. are required to meet this minimum octane rating.
Use of a fuel too low in octane (a measurement of anti-knock quality) will result in spark knock. Since many factors affect operating efficiency, such as altitude, terrain, and air temperature and humidity, knocking may result even though the recommended fuel is being used. If persistent knocking occurs, it may be necessary to switch to a slightly higher grade of unleaded gasoline. Continuous or heavy knocking may result in serious engine damage, for which the manufacturer is not responsible.
Your car's engine fuel requirement can change with time, due to carbon buildup, which changes the compression ratio. If your car's engine knocks, pings, or runs on, switch to a higher grade of fuel, if possible, and check the ignition timing. Sometimes changing brands of gasoline will cure the problem. If it is necessary to retard timing from specifications, don't change it more than a few degrees. Retarded timing will reduce power output and fuel mileage, and will increase engine temperature.