CAUTION: Volkswagen does not approve the use of more than 5% biodiesel on TDI vehicles sold in North America, and does not approve the use of straight vegetable oil (SVO) or waste vegetable oil (WVO) at all. Use of unapproved fuels is considered "misfueling". If there is a fuel system related failure, and misfueling is found to be involved, any damage may not be covered under warranty.

If you wish to use such fuels on an "I am my own warranty" basis, you should be made aware of the risks involved. The process of making biodiesel inherently involves alcohol, water, glycerine, lye, and byproducts. Even tiny amounts of contamination can result in severe damage to the high-pressure fuel injection components. Biodiesel must conform to ASTM standards. "Home-brewers" generally have no practical way to check whether their biodiesel conforms to standards.

Vegetable oil that has not been chemically transformed into biodiesel generally has a viscosity which is much higher than the fuel injection system is designed for, which puts more stress on the system. This is of particular importance on "common-rail" or "pumpe-duse" fuel injection systems (i.e. all 2004 and later North American models), and the conventional wisdom is that vegetable oil is not to be used in these engines. People wishing to install "kits" to fuel their vehicle with vegetable oil are strongly urged to take into account whether a particular "kit" is tailored specifically for their particular vehicle and engine, whether the kit manufacturer offers a warranty on only the kit or on the complete converted engine, and whether the kit has any type of independent third-party approvals. Absence of a "kit" for a particular engine should be interpreted as that engine being unsuitable for operation on vegetable oil.