I've just come across a site in german that tells of the need to take it steady on the biodiesel front, don't use to much or you might damage the car, very few manufacturers say its safe to use, (except for VW and Skoda).
If you fancy giving your German a run round the block check out consumerblog.de
Handily, the site also tells you how to find out how to tell if you're putting biodiesel in your tank.
(They put stickers on the pumps!)