Ford Mercury Villager 1997 Owner's Manual page 313

Maintenance and Care
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
Do not be concerned if your vehicle sometimes knocks lightly.
However, if it knocks heavily under most driving conditions
on the recommended octane 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 a qualified service technician.
The American Automobile Manufacturers Association (AAMA)
issued a gasoline specification to provide information on high
quality fuels that optimize the performance of your vehicle.
We recommend the use of gasolines that meet the AAMA
specification if they are available.
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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