Ford 1998 Contour Manual page 163

Maintenance and care
Octane recommendations
Your vehicle is designed to use
MINIMUM OCTANE RATING
regular gasoline with an (R+M)/2
(R+M)/2 Method
87
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.
Do not be concerned if your vehicle
sometimes knocks lightly. However,
if it knocks heavily under most
driving conditions on the
recommended octane, see your
dealer or a qualified service
technician to prevent any engine
damage.
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