Mercury 1996 Sable Manual page 293

Typical octane rating label
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.
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