Ford 2003 Mustang Owner's Manual page 169

Octane recommendations
3.0L V6 Vulcan and 3.0L DOHC V6 Duratec
engines
Your vehicle is
designed to use
"Regular" unleaded
gasoline with an
(R+M)/2 octane rating of 87. We do not recommend
the use of gasolines labeled as "Regular" that are
sold with octane ratings of 86 or lower in high
altitude areas.
Do not be concerned if your engine sometimes
knocks lightly. However, if it knocks heavily under
most driving conditions while you are using fuel with
the recommended octane rating, see your dealer or
a qualified service technician to prevent any engine
damage.
3.4L-32V V8 SHO engine
Your vehicle is
designed to use
"Premium" gasoline
with an (R+M)/2
octane rating of 91 or higher for optimum
performance. The use of gasolines with lower octane
ratings may degrade performance. We do not
recommend the use of gasolines labeled as
"Premium" in high altitude areas that are sold with
octane ratings of less than 91.
Do not be concerned if your engine sometimes
knocks lightly. However, if it knocks heavily under
most driving conditions while you are using fuel with
the recommended octane rating, see your dealer or
a qualified service technician to prevent any engine
damage.
Maintenance and care
87
(R+M)/2 METHOD
91
(R+M)/2 METHOD
169