I like the idea of the bio-diesel fuel. Re-refined waste oil from fast food joints.
the only problem is that it is hard to find a place to refuel. the beauty of the bio-diesel system is that there are no modifications to a true diesel engine.
The same engine can run either of the fuels or a mixture of the 2. so you are running low on fuel, you can still stop and fill up with real diesel at the pump.
http://www.biodiesel.org/