Biodiesel may be almost impossible to avoid in Illinois, as I believe they mandate 20%. I think that is why VW allows up to 20% for TDI's in Illinois. Here is the deal with biodiesel in modern TDI's with advanced exhaust after treatment. The DPF(diesel particulate filter) removes the soot from the exhaust and helps to give our TDI's their clean reputation. Every once in awhile the cars computer tells the injectors to squirt some extra fuel to help burn off and remove the soot from the DPF. Some of this extra fuel blows past the piston rings and ends up in the oil. Straight diesel that ends up in the oil from this process is no problem as it boils away at engine operating temps, but biodiesel has a higher boiling point and tends to accumulate in the oil, thinning it out and making it less lubricating. This can cause excessive wear. VW says you can run 5% without issues and allows up to 20% for states that have a biodiesel mandate calling for 20%. 5-20% should be fine for your car, just check the oil occasionally, if the oil level rises over time that indicates that you are getting fuel in the oil. This is mentioned in your owners manual under checking your oil, if you haven't read the manual then I highly recommend it
You really have nothing to worry about, buy good quality fuel and drive more and worry less
Sorry for the long post, sometimes I get rambling on.