The TDI forum is a great resource for the additive discussion. In fact, it's been more than beaten to death.
I use OptiLube XPD (about 4 ounces per fill up) with every tank. If you do a search on "Spicer Report" it may shed some light on the subject. Though I wouldn't take the report as pure truth, it does show that some products are better than others. Biodiesel as an additive has the best lubricity, but some of the additives like the Opti add demulsifiers (help with water issues), cetane booster and anti gelling compounds.