Ford Windstar Owner's Manual page 312

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. Using leaded fuel is prohibited by
federal law and could damage your vehicle. The damage may
not be covered by your warranty.
Vehicle inspection programs may detect misfueling. Under these
circumstances, you may be required by law to make costly
repairs at your own expense.
Octane recommendation
Your vehicle is designed to use regular unleaded gasoline with
an (R+M)/2 octane rating of 87. We do not recommend
gasolines labeled "regular" in high altitude areas that are sold
with octane ratings of 86 or even less.
Typical octane rating label
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