My dad actually had some experience with wood gas back during WWII, a lot of the German civillian cars were converted to run on it as the "real" fuels were being used up by the military. Two main problems were you needed a good filter to keep the solids in the smoke from congealing inside the engine, and accelleration was anemic at best.
Diesel engines (at least the older ones) will run on just about any type of light oil, newer electronically controlled ones might not do so well on some fuels. Even if the used fryer oil does suck in winter temps, no one sez you gotta use it year round. Running for "free" throughout the summer is sure a heck of a lot cheaper than buying diesel all year long.