I read quite a bit about this when I first got my car, and the reason why it's not recommended to use above B5 is due to the regen cycle for the DPF. If I remember correctly, The way it works on the TDIs is diesel fuel is injected into the cylinder during the exhaust cycle, which is at a high enough temperature to combust the diesel without compression. However, biodiesel has a higher flash point than petrol diesel, so you don't get a complete combustion and the uncombusted diesel Supposedly leaks out and contaminates your oil. I think if you use biodiesel you are supposed to just change your oil more frequently, which negates the point of biodiesel from an environmental standpoint. The way other Diesel engines get around this is they inject the diesel into the exhaust pipe, (which I guess is still hot enough to combust the fuel), not the cylinder, so you don't dilute your engine oil. Not sure why VW decided to inject it into the cylinder... probably to cut cost on a separate injection system. But long story short, any kind of biodiesel should be okay for one fillup every now and then, I suppose ideally after a regen cycle and as low %biodiesel as possible, but prolonged use will mess up your engine unless you flush the engine oil regularly.
