1997 Lincoln Mark VIII Owner's Manual - Page 251

Octane recommendation Your vehicle is designed to use premium gasoline for optimum performance with an (R+M)/2 octane rating of 91 or higher. Gasolines with lower octane ratings can be used, but performance and fuel economy may decrease. We do not recommend gasolines labeled as "premium" in high altitude areas that are sold with octane ratings less than 91. Fuel quality If you are experiencing starting, rough idle or hesitation problems try a different brand of fuel. If the condition persists, see your dealer or a qualified service technician. The American Automobile Manufacturers Association (AAMA) issued a gasoline specification to provide information on high quality fuels that optimize the performance of your vehicle. We recommend the use of gasolines that meet the AAMA specification if they are available. It should not be necessary to add any aftermarket products to your fuel tank if you continue to use a high-quality fuel. Typical octane rating label Do not be concerned if your vehicle sometimes knocks lightly. However, if it knocks heavily under most driving conditions on the recommended octane fuel, see your dealer or a qualified service technician to prevent any engine damage. 246