1978 Cadillac DeVille - Diesel question

Diesel fuel used in cars. Is this different diesel fuel used in farm tractors?

Not for a 1978 model diesel car it isn’t.

Only difference I’m aware of, the road tax wasn’t paid on farm tractor diesel.

Different (for a '78)? No. Legal? No.

I think farm diesel used to have a dye in it . . . it still might, for all I know

Long-haul truckers could face a HEFTY fine if they were caught with farm diesel in their tanks

Yes, it’s different because of the taxes paid. Highway use diesel, off road use diesel, and home heating oil are all taxed differently and are dyed differently to identify their use and for leak detection.

When I had my own shop that had an oil-fired furnace I simply filled the tank by running 5 gallon cans from the diesel pumps out front. Whenever the furnace needed service the repair tech always commented that I wasn’t using the proper oil. It apparently is dyed to identify leaks.