A multifuel's pump automatically adjusts for changes in the viscosity of the fuel. Thats what allows it to run almost any mix of thick and thin fuels, such as motor oil. Personally I would run a decent mix of diesel or gas in there to thin it a bit, to make it easier to get through lines, filters, etc. I'm probably just being overcautious though.
I even run waste motor oil in my 4bt powered Durango. I just never run more than 25% motor oil in the tank, usually more like 10-15%. It doesn't seem to smoke any more and its a good way to recycle the oil into something usable.
The key things about waste oil are to make sure its CLEAN, or PURE is a better word. No water, antifreeze, synthetics, chunks, etc. If you are a mechanic and can read the customer's vehicle and know its a well maintained vehicle, keep that oil. Use a separate container for your Deuce's fuel/oil to keep contaminated oil out of it. Most people just dump it in the tank and go. I like to filter it first...check my above post for my method. My father (Beaubeau on here) has been running his Deuces on all of his waste motor oil, trans fluid, hydraulic fluid, etc. When he had to use a Deuce to go to PA (500 miles each way) to tow back a 5-ton, he left N.H. with a full fuel tank and second tank in the body full of whatever he had around. Free fuel to PA!
Depending on what you run, you may notice changes in the power of the truck. Some oils have less BTU available to make power with and will reduce power and fuel mileage a bit. But if the fuel is free, its not hard to live with 1 or 2mpg less mileage!
Jim