FORD 2010 Flex v.2 - Manual (Page 364)

Your vehicle was not designed to use fuel or fuel additives with metallic
compounds, including manganese-based additives.
Note: Use of any fuel other than those recommended may cause
powertrain damage, a loss of vehicle performance, and repairs may not
be covered under warranty.
Octane recommendations
3.5L V6 engine
Your vehicle will run normally on 87 octane regular fuel. Premium fuel
will provide improved performance.
3.5L V6 EcoBoost engine
Your vehicle is designed to run on regular fuel with an octane rating of
87 or higher. For best overall performance, premium fuel with an octane
rating of 91 or higher is recommended. The performance gained by using
premium fuel will be most noticeable in hot weather or in severe duty
applications such as towing a trailer.
Some stations offer fuels posted as
ЉRegularЉ with an octane rating below
87, particularly in high altitude areas. Fuels with octane levels below 87
are not recommended for either engine. Do not be concerned if your
engine sometimes knocks lightly. However, if it knocks heavily while you
are using fuel with the recommended octane rating, see your authorized
dealer to prevent any engine damage.
Fuel quality
If you are experiencing starting, rough idle or hesitation driveability
problems during a cold start, try a different brand of unleaded gasoline.
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.
87
(R+M)/2 METHOD
Maintenance and Specifications
364
2010 Flex (471)
Owners Guide, 2nd Printing
USA (fus)