Dear Tom and Ray: I heard an interview recently on NPR's "Fresh Air." Terry Gross was interviewing Willie Nelson, who was talking about how his wife bought a diesel car a few years back and started using biofuel in it (recycled grease and oil, reconstituted into vegetable oil -- that's biofuel?). Then he said he bought a Mercedes diesel and started using it in that car. Is biofuel usable in all diesel engines? -- Paul

Tom: One type of biofuel is, Paul. It's called biodiesel. And Willie Nelson not only uses it, he sells the stuff. He calls it Bio Willie.

Ray: There are two basic types of plant-oil fuels, Paul. The first type is biodiesel, which can be used in almost any dieselpowered car. That's usually soybean or rapeseed oil that's chemically altered to remove the glycerine, and then mixed with traditional diesel fuel.

Tom: The regular diesel fuel is in there to make the mixture easier to start, and to keep it from getting gloppy at lower temperatures.

Ray: You'll see it listed for sale as B5, B20 or B100 biodiesel. B5, for instance, means it's 5 percent biodiesel and 95 percent diesel fuel. B5 is universally considered safe for all diesel engines. Lots of people say B20 is fine, too.

Tom: The jury is still out on higher concentrations. There's some concern that in older engines, such as from the early 1990s, before synthetic fuel lines were widely used, biodiesel might break down rubber components in the fuel system, so it's worth having rubber components inspected after six months of use. But there are a lot of people who say they use B20 in the winter and B100 in the summer without any problem at all.

Ray: So that's biodiesel. Then there's plain old vegetable oil -- without diesel fuel mixed in. That can either be manufactured directly from plants or collected as waste oil from the fryolator at your local Burger Queen.

Tom: But that's not something you can use in just any car. It congeals much too easily. Cars have to be modified significantly to run on straight vegetable oil (also called SVO).

Ray: You need to have a separate tank for regular diesel fuel. You use the diesel to start your engine and warm it up. The engine's coolant would then warm up the SVO. Once it's warm, you can switch to using the SVO as fuel. But then you have to switch back to regular diesel fuel before you shut it off, to make sure there's no SVO left in the fuel lines when you shut it down. Otherwise, the lines will clog up like my Uncle Ciccione's arteries.

Tom: So SVO is still for the wacko fringe (go ahead, write to me and complain, fellas; I can take it), and not the average guy who wants to do a little something easy to improve the environment.

Ray: Biodiesel will do a little bit to improve things. It does burn cleaner than standard diesel fuel. And assuming the "bio" part is grown in the United States, it helps promote energy independence.