This is the document I was talking about:
http://biodiesel.org/pdf_files/fuelfactsheets/Use_of_Biodiesel_Blends_above_%2020.pdf

It explains the potential dangers of using more than B20.

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2004 VW Jetta TDI (manual)

Past MB's: '96 E300D, '83 240D, '82 300D, '87 300D, '87 420SEL