I'm all for recycling, but I would be a little hesitant to switch to restaurant 'grease' before doing a lot of research.
Todays highway diesel is a blend that contains additives for lubricity, anti-corrosion, and cetane levels along with other elements only a chemical engineer would care to know about.
I also own a VW TDI diesel, and monitor a very active VW diesel web page called Fred's TDI @
http://forums.tdiclub.com/. I have seen a lot 'grease' enthusiast come and go, and several used 'grease' kits for sale, which makes me a little suspect. I recommend you search some of the 'fuel' threads before making the leap to 'french fry fumes'.
Personally speaking, I would be hesitant to exceed 5-10% Biofuel until more is known of its long term consequences to injectors, fuel pumps, computer sensors and lubricity- in other words 'you first'.
John