Make sure you filter the used motor oil and know where it comes from. Used motor oil and biodiesel in a 50/50 mix works too. I ran used motor oil in my rig for a couple months and a few thousand miles and now I run straight biodiesel. I had my pump rebuilt and even though I filter to 4 microns it still had junk that was not to kind to the close internal tolerances of the pump (used motor oil, age, miles and the idiot previous owner killed it, not JUST used motor oil, but it did play a part in its rapid decline at the end)
A plus to bio here in oregon is the fuel tax kickback, we get 50 cents back per gallon with reciepts when we do taxes, Im looking forward to it.
punk