When making Biodiesel the final step is washing the fuel. This is when I add the old engine oil. The water (yes water) pulls the dirt, metal, acid and other bad things to the bottom. Then the fuel is filtered through two 1 micron fuel filters. Here is a link to a prossess close to what I use:

http://journeytoforever.org/biodiesel_aleksnew.html

If you are running a fleet of vehicals the used oil piles up fast and is a good lube, tranny fluid is by far better. We have a 55 gallon drum and all the suspension fluid from the race bikes, tranny fluid and use motor oil goes into it and is added at the washing prossess of making the biodiesel. All water is removed from the fuel in the end and the fuel is almost clear.