What type of gas does a 1978 F250 Ford truck use?

A 1978 Ford F250 could have used either leaded regular gasoline or, less commonly, unleaded regular gasoline. The engine and specific options determined the fuel type. Many engines of that era were designed for leaded fuel, but some might have been offered with modifications allowing the use of unleaded fuel. Checking the owner's manual (if you have it) or the sticker under the hood would be the definitive way to know for certain which fuel type was specified for a particular truck.