I've kicked around the fryer oil injection systems, and making bio diesel. Neither is a good option in cold climates. Bio diesel gels up around 25F without any additives. Its was -20F for a month straight this last winter. This new low sulfur crap is so clean that there isn't much of an advantage to bio diesel in the enviroment department either. But if you make it your self you can get the cost down around 35-45 cents a gallon supposedly.