Will, if you look at
www.greasecar.com that's essentially what they have been doing in cars for years. Installing a second tank for "WVO" aka waste vegetable oil, the stuff the restaurants have to pay to dispose of. Yes, your diesel can burn anything "but". With used motor oil you have to worry about toxins in the exhaust. And with vegetable oils, they congeal when cool, so the greasecar folks use an electric heating line to make sure the second fuel tank gets heated nice and warm. You start on real diesel--when it is not hot out--and switch back to real diesel to flush the lines again before you shut down.
Herr Doctor Professor Whatever Rudolph Diesel invented his engine to run on PEANUT OIL not petroleum products, so this is not hard to do. If you've got the space for a second tank, and a little more human control for the switching.
And remember--that other oil source needs to be filtered VERY well.