I like the whole WVO thing but from what I've read (no first hand experience here) your maintenance cost can increase dramatically. The most susceptible components are the injection pump and injectors. Even in a good scenario injectors have to be cleaned, rebuilt pop tested and re balanced every year. Its easy if you have the equipment and know how. The injection pump is another animal all together. This is the most complex and expensive component on the engine and that WVO is flowing right through it at 2000psi. You better have very high confidence in your filtering and fuel heating ability nevermind the deposits that the WVO leaves behind.
Running WVO as a reliable long term fuel requires a big up front investment.
Autospeed article WVO
Or you can pour it in and do it for show to impress all your friends.
I admire the people doing this, I'm all for it. I wish I had the time, space, resources to convert WVO into fuel but it doesn't make economic sense for me. Even free fuel isn't really free. I have burned used motor oil in my diesel after allowing it to settle for a month then filtering but this only represents a tiny fraction of all the diesel fuel I use.
Also I think its disengenuous to say you are getting 80-100mpg in your MB even with the qualifiers in your post. Even if you get the WVO for free you still spent time, storing and filtering the fuel.
cheers