Ford Windstar Owner's Manual page 313

In some parts of the country, "regular" grade fuels are sold
with octane ratings of 86 or even less especially in high altitude
areas. We recommend that you do not use these fuels. Always
use a fuel with an octane rating of at least 87, even if it is sold
as a "midgrade" or "premium."
Do not be concerned if your vehicle sometimes knocks lightly.
However, if it knocks heavily under most driving conditions on
the recommended octane rated fuel, see your dealer or a
qualified service technician to prevent any engine damage.
Fuel quality
If your vehicle has problems starting, rough idle, or hesitation
problems when the engine is cold, it may be caused by fuel
with low volatility. 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.
minimizing deposits in fuel injectors, combustion chambers
and on intake valves.
assuring quick starting and smooth warm-up.
protecting the fuel system from vapor lock in very hot
weather, which causes the engine to hesitate or stall and
makes restarts difficult.
protecting your fuel system from rusting.
preventing the fuel lines from freezing.
preventing the fuel system from "gumming up" when your
vehicle is not used for long periods of time.
It should not be necessary to add any aftermarket products to
your fuel tank if you continue to use a high-quality fuel.
Maintenance and Care
313