Biodiesels are methyl or ethyl combustibles from vegetable oils, waste cooking oil, and can even be derived from animal fats. Theoretically, any vegetable oil can be used as a biodiesel source, but if you compare the price per liter at the grocery store, you will find there is no real savings (unless the price of a barrel of oil gets a lot higher than it is currently). Environmentally however, there is some advantage to using biodiesel blends in older diesels.
Biodiesel blends tend to reduce visible smoke and reduce hydrocarbons and carbon-monoxide emissions in older diesel engines. This is an advantage if used in an urban environment with a pre-2005 diesel, but in the newer low-emission diesel engines, biodiesel does not have any practical advantage.

Currently, biodiesel is usually blended with petroleum-based diesel fuel. B5 or 5% diesel blend is now sold in many areas of Canada and the U.S. 5% biodiesel will not cause any known problems with injection systems, but in some areas up to 20% or more biodiesel has been added. As of summer 2007 there was still no certification or specification for biodiesel blends over 5%. There are even some brave folks running 100% biodiesel in their VW diesels, although one would have to ask why, since this is more expensive than petroleum-based diesel, and results in a 5%-7% drop in power.

Other dis-advantages to high amounts of biodiesel include: poor flow (high viscosity) at low temperatures, more water content in the fuel, and increased microbial growth in fuel tanks.

The truck manufacturers' official position on the use of biodiesel fuels is available at
www.enginemanufacturers.org