Choosing The Right Fuel; Octane Recommendations - Ford 1999 F-350 Owner's Manual

Maintenance and care
When refueling always shut the engine off and never allow
sparks or open flames near the filler neck. Never smoke while
refueling. Fuel vapor is extremely hazardous under certain conditions.
Care should be taken to avoid inhaling excess fumes.
The flow of fuel through a fuel pump nozzle can produce static
electricity, which can cause a fire if fuel is pumped into an
ungrounded fuel container.
Use the following guidelines to avoid static build-up when filling an
ungrounded fuel container:
• Place approved fuel container on the ground.
• DO NOT fill a fuel container while it is in the vehicle.
• Keep the fuel pump nozzle in contact with the fuel container while
filling.
• DO NOT use a device that would hold the fuel pump handle in the fill
position.

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 compounds containing MMT.
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 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
196
87
(R+M)/2 METHOD