My '93 E350 shuttle and my '90 F350 flatbed are WMO fueled.
Here are a few key things to remember that will keep it "mostly" problem free.
2 tanks are nearly a must. Starting (and warm up), and shutting down on pump diesel keeps the injectors clean and keeps coking to a minimum. Oil has a higher potential energy... but burns slower then diesel fuel and takes more heat to burn completely. Running a cold engine on waste oil is a good way to cause problems.
Clean oil. We centrifuge our oil using a 55gph centrifuge from PA Biodiesel. It is powered by an old power steering pump with a pressure relief (we run the system at around 90psi for 48 hours). Some have had luck with filters... we started with filters. When we got our centrifuge, we ran a batch of filtered oil through it for giggles and ended up with a bowl full of sludge and visible metallic particles. With the cost of filter replacement... its cheaper and easier to go with a centrifuge from the get-go.
Get the oil thin. To do that, we batch in a 55 gallon drum. In that drum goes 5-10 gallons of RUG (regular unleaded gasoline) and 45-50 gallons of WMO (waste motor oil, atf, hydraulic oil, etc). This is our centrifuge drum as well.. centrifuge is mounted on top of this drum and cycles the contents continuously for 48 hrs.
With this combination, we've ran through hundreds of gallons of W90 (90%oil, 10% RUG) without much issue. We keep a fairly large stockpile of waste oil on hand (~2k gallons) that allows us to settle the oil for long periods.. then pull from the top of the storage tanks into the centrifuge drum.
You can get waste oils for free from many places. Our best sources are small farms that don't want to pay to haul off waste oil from servicing their equipment each year. We'll typically set them up with a 275g tote and they will fill them up a couple times a year.
Here are some pics from our setup.
The centrifuge drum with centrifuge on top (blue canister is the 'fuge)
, on Flickr
Old electric motor (with overtemp protection) belt driving a power steering pump from a ford focus
, on Flickr
Ashcroft pressure switches (one for high pressure... one for low pressure.. operating window of ~8psi)
, on Flickr
This setup has worked great for us. Now that the bus will be waste oil fueled, we are going to step up our game come spring. We'll be getting a bigger 'fuge and batching 200-250 gallons at a time.
On a side note - Due to cost of replacement parts, etc.. I would NOT run WMO on a modern diesel. I can replace the ENTIRE injection system on my old IDIs for the cost of 2-4 injectors on a modern diesel. Not worth the risk. Also, most modern diesels use fuel opacity sensors which wont allow them to run on black diesel. The 7.3 powerstroke is the newest engine that I've heard of people having good luck with waste oils on.
Burning waste oils is also one of the few EPA certified ways of properly disposing of it. In a mechanical diesel, when up to operating temp.. it is being injected under high temp and high compression and it burns fairly clean. On my old IDIs, there is no noticeable difference in visible emissions or smells when running on waste oil.