Ford Mustang Owner's Manual page 185

Maintenance and Specifications
The fuel system may be under pressure. If
the fuel filler cap is venting vapor or if you
hear a hissing sound, wait until it stops before
completely removing the fuel filler cap. Otherwise,
fuel may spray out and injure you or others.
If you do not use the proper fuel filler cap,
excessive pressure or vacuum in the fuel
tank may damage the fuel system or cause the fuel
cap to disengage in a collision, which may result in
possible personal injury.
Choosing the right fuel
Use only UNLEADED FUEL. The use of leaded fuel
is prohibited by law and could damage your vehicle.
Do not use fuel containing methanol. It can damage
critical fuel system components.
Your vehicle was not designed to use fuel or fuel
additives with metallic compounds, including
manganese-based additives.
Repairs to correct the effects of using a fuel for
which your vehicle was not designed may not be
covered by your warranty.
Octane recommendations
Your vehicle is
designed to use
"Regular" unleaded
gasoline with pump
(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.
2004 Mustang (mus)
Owners Guide (post-2002-fmt)
USA English (fus)
87
(R+M)/2 METHOD
185