(FORTUNE Magazine) - FORGET "WHISKEY RIVER." WILLIE NELSON has a new fuel of choice. He is rollin' toward California "about an hour west of Large Bird, N.M.," on his tour bus. That's apropos, actually, because he's talking about biodiesel, a mix of regular diesel and refined vegetable oil that happens to be the fuel in the bus. Nelson is America's foremost proponent of biodiesel, most visibly as the marketer of a brand called BioWillie. It's for sale at 12 gas stations, the majority in Texas, including his own truck stop, Carl's Corner in Hillsboro, south of Dallas.

Nelson's blend, B20, is a mix of 80% diesel and 20% mostly vegetable oil that, in the case of BioWillie, often comes from soybeans. Produced through a process called transesterification, biodiesel can be rendered from peanuts, rapeseed, and other plant products, or from leftover oil or lard. How'd Willie get onto it? "My wife heard about some guy making it from restaurant grease traps out in Maui, where we live," he says.

Nelson, who's on the board of a tiny company called Earth Biofuels (its website plays "On the Road Again"), says biodiesel is a three-way winner. "It's less harmful to the environment, it will help family farms by creating more demand for agricultural products, and we don't need to get involved in wars overseas to find oil."

There are a few negatives, though. Biodiesel tends to congeal in winter much more easily than regular diesel--at around seven degrees for B20--but Nelson says there are new additives to mitigate that. Also, some critics say biofuel production could eventually sap our food supply. And a few contend that biodiesel actually requires too much energy, fertilizer, and farm equipment to produce. Willie has little patience for these views. Anyone who thinks such thoughts is "an idiot, biased, or stupid," he says. "Truckers love it, and farmers can grow it easily." As for biodiesel diminishing our food stocks, "That's horseshit," says Willie. And it's true that the latter point would seem to fly in the face of our obesity epidemic. How big could biodiesel get? "We can reduce our dependence on imported oil by 10% with biodiesel," says Nelson. "Who wouldn't want to do that?" Good question.