* Halogen: If your F-350 has halogen headlights, it likely uses a 9007 bulb (also sometimes called a HB5). This is a dual-filament bulb handling both high and low beams.
* HID (Xenon): If your F-350 has HID headlights, you'll need to check your owner's manual for the specific bulb type. HID bulbs are not interchangeable like halogen bulbs. They are often a D2S or similar type, and should be replaced with the same type and wattage.
To be absolutely sure, check your owner's manual or the sticker inside your glove compartment. The sticker should detail the specific bulb type. Failing that, visually inspecting the bulb itself (when the headlights are off, of course!) might reveal the bulb type printed on the glass.
