1997 Ford Aerostar Owner's Manual - Page 263

Choosing the Right Fuel Use only UNLEADED FUEL. The use of leaded fuel is prohibited by law and could damage your vehicle. The damage may not be covered by your warranty. Your vehicle was not designed to use fuel containing manganese-based additives such as MMT. Additionally, vehicles certified to California emission standards (indicated on the underhood Vehicle Emissions Control Information label) are designed to operate on California reformulated gasolines. If California reformulated gasoline is not available when you refuel, your vehicle can be operated on non-California fuels. However, even though your engine will perform adequately on other gasolines, the performance of the emission control devices and systems may be adversely affected. Repair of damage caused by using a fuel that your vehicle was not designed for may not be covered by your warranty. Octane recommendation Your vehicle is designed to use regular gasoline with an (R+M)/2 octane rating of 87. We do not recommend gasolines labeled as "regular" in high altitude areas that are sold with octane ratings of 86 or even less. Typical octane rating label 264