I found this clip from Mythbusters where they use a Mercedes with a diesel engine and fuel it with used oil from a commercial fryolator. I dont know how clean burning this is and its mileage is slightly less than diesel, but if this went mainstream it would take a huge amount of demand off of the gasoline, not to mention reduce the amount of cooking oil that is simply tossed away. The potential here is pretty exciting.
Im not sure how to embed here, or if we are enabled to do so, but here is the link.
http://www.youtube.com/watch?v=YOGIh3yRltE