I just purchased my 2010 Jetta Sportwagen TDI this week. I did quite a bit of research on using biodiesel with it before purchase but since the post-injection DPF filters are so new (only out since 2009) there's not much "real-life" information out there. Basically, VW warranties the engine for ASTM-certified B5. When I contacted biodiesel wholesalers in my area they all encouraged me to use up to a B20 blend and I've found some sources online that seem to think that would be OK but it's a risk as the use of biodiesel with the DPF system can deplete oil, clog the filter and reduce efficiency. I've sourced commercial B5 in my area and intend to use that and have my oil and filter checked more frequently. I may use up to a B20 blend in the future but am taking baby steps right now since I really can't afford to replace the engine if it goes. Here are some links that might help you decide for yourself:
Diesel particulate filter DPF and Adblue fluid FAQ
Understanding the Post-Injection Problem
VW world’s 1st automaker to warrant B5 biodiesel
2009, 2010, and 2011 VW Jetta Sportwagen (Golf wagon) TDI buying checklist and FAQ (check out the Biodiesel section)
Let me know if you find other info. I really want to use biodiesel but I don't want to risk recking brand new car. :/
-Rebecca