Ford Mustang Owner's Manual page 294

Maintenance and Specifications
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 authorized dealer to
prevent any engine damage.
Octane recommendations (5.0L V8 engine)
Your vehicle will run normally on 87 octane regular fuel without
damaging the engine, but premium fuel with an octane rating of 91
(R+M)/2 or higher is recommended for best overall performance.
For Shelby GT500 octane requirements, see the Shelby GT500
Supplement.
Fuel quality
If you are experiencing starting, rough idle or hesitation driveability
problems during a cold start, try a different brand of "Regular" unleaded
gasoline. Premium unleaded gasoline is not recommended for vehicles
designed to use "Regular" unleaded gasoline because it may cause these
problems to become more pronounced. If the problems persist, see your
authorized dealer.
Do not add aftermarket fuel additive products to your fuel tank. It
should not be necessary to add any aftermarket products to your fuel
tank if you continue to use high quality fuel of the recommended octane
rating. These products have not been approved for your engine and
could cause damage to the fuel system. Repairs to correct the effects of
using an aftermarket product in your fuel may not be covered by your
warranty.
Many of the world's automakers approved the World-Wide Fuel Charter
that recommends gasoline specifications to provide improved
performance and emission control system protection for your vehicle.
Gasolines that meet the World-Wide Fuel Charter should be used when
available. Ask your fuel supplier about gasolines that meet the
World-Wide Fuel Charter.
Cleaner air
Ford endorses the use of reformulated "cleaner-burning" gasolines to
improve air quality, per the recommendations in the Choosing the right
fuel section.
Running out of fuel
Avoid running out of fuel because this situation may have an adverse
effect on powertrain components.
294
2011 05+ Mustang (197)
Owners Guide, 1st Printing
USA (fus)