Currently, buying B20 is a bit of a roll of the dice. Researchers from Wood's Hole Oceanographic institute were testing biodiesel to evaluate the effects of spills on marine environments. They bought samples from a dozen distributors around the country and found the concentrations of biodiesel in B20 (nominally 20 percent bio and 80 percent petroleum) ranged from a low of 10 percent to as much as 74 percent (!). While using more biofuel may be desirable in many respects, if a vehicle isn't prepared for such high concentrations, it might cause serious damage. Homebrew biodiesel or blends are fine if you know what you're getting into, but most people need a reliable source of fuel and don't want to have to pay for expensive repairs. The U.S. Defense Department is a major consumer of biodiesel and of the samples tested only ten percent met their standard which may cause them to curtail purchases.
[Source: World Business Council on Sustainable Development, via EcoGeek]