I know some kids whose van runs entirely on vegetable oil that they take from dumpsters behind fast food restaurants. This of course instantly caught my attention. It's probably not a great idea since I'm selling my car in less than six months, but I was curious about the process.
What I learned is that vegetable oil actually works fine in some diesel engines. One of the main problems is that vegetable oil doesn't work very well when it's cold due to it's thickness. However for under $1000 one can install a second engine and heater so that they can use diesel oil until the vegetable oil heats up. Also used oil needs to be filtered. This filtering process basically involves adding methanol and lye to the oil. But overall one will save a lot of money.