Highway Use Tax on Alternative Fuels

BACKGROUND: I have a client who uses diesel vehicles for both business (self-employed) and personal purposes. He has converted the fuel use to bio-fuel. He gets used vegetable oil from restaurants.

QUESTIONS:

1. Is he responsible to paying a highway use tax on Form 720?
2. If so, how/where is it reported on the form?
3. Is there any registration requirements?
4. Are there any other tax implications?

Thanks for any help.