Sure you can use it. The worst you can do is cause an engine failure.

The warning label to not use it is simply because there is no spec for what bio-diesel is, any particular bio-diesel fuel can be anything from a clean commercial fuel to a witch's brew mixed up in a garage. BMW is simply saying that you are on your own in the event of consequential damage. If you feel that the rewards outweigh the risks, by all means give it a go.

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2007 X3 3.0si, 6 MT, Premium, White

Retired:
2008 535i, 6 MT, M Sport, Premium, Space Grey
2003 X5 3.0 Steptronic, Premium, Titanium Silver
2002 325xi 5 MT, Steel Grey
2004 Z4 3.0 Premium, Sport, SMG, Maldives Blue