Ford 1996 Windstar Manual page 311

Servicing Your Windstar
We do not recommend gasolines labeled "regular" that are sold
with octane ratings of 86 or even less in high altitude areas.
Do not be concerned if your vehicle sometimes knocks lightly.
However, if it knocks heavily under most driving conditions on
the recommended octane rating fuel, see your dealer or a
qualified service technician to prevent any engine damage.
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 qualified service technician.
It should not be necessary to add any aftermarket products to
your fuel tank if you continue to use a high-quality fuel.
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