Ford 1996 Windstar Manual page 310

A venting fuel tank is not an abnormal condition. It may be
caused by:
too volatile a fuel for the weather conditions. Service stations
sometimes sell winter grade fuel in the summer.
pulling a heavy load on hot days, or at high altitudes.
extended periods of idling with the engine RPM increased
above the normal idle range.
parking the vehicle in full sunlight for extended periods on
extremely hot days.
Choosing the Right Fuel
Use only UNLEADED FUEL in your vehicle. Using leaded fuel
is prohibited by federal law. Any vehicle damage may not be
covered by your warranty.
Octane recommendations
Your vehicle is designed to use regular gasoline with an
octane rating of 87.
Typical octane rating label
310