I don't know if people are still reading this thread, but I'll give it a go.
There are basically a few different types of waste oil that could be used as fuel extender.
Used sump oil
Waste cooking oil
Used transmission oil.
Hydraulic fluid and A/T fluid
I would forget the last two because of the high content of additives in it.
Used sump oil. Most old timer truck drivers poor their sump oil back in the fuel tank. Yet this amounts to less than 10% mix. I wouldn't do it.
However there is a way to do it safely, and that is by getting the oil through a centrifuge first. Forget filtration, the best filters can at best stop 5 micron particles. a centrifuge gets rid of particles down to one micron and can be cleaned and up it goes again.
Considering that an Alfa Laval "Emmie" costs $30,000, this seems a lost cause, yet I found a cheap alternative http://www.simplecentrifuge.com
Centrifuge your used mineral oil, and mix at a guess not more than 30% depending on what engine you are using. Older engines take better to thicker fuel than new one.
On the subject of cheaper fuel, a centrifuge should also make it possible to use heavier diesel designed for bigger engines. The centrifuge will eliminate the particles of tar that would otherwise ruin your injectors and pump.
Waste vegetable oil or even new vegetable oil, is another matter.
You can run a diesel engine on straight vegetable oil if you take a series of precautions and convert your engine to use this fuel. It implies the use of a dual fuel system to start and stop the engine on mineral diesel and also a system to heat up the fuel. There is ample information on the net if you search "straight vegetable oil".
An alternative to SVO is the much tooted Bio-Diesel. It involves converting the vegetable oil into a fuel that has similar viscosity to Diesel and for that it is necessary to put the waste or new vegetable oil through a process that includes decanting, a chemical reaction with methanol and Potassium or Sodium hydroxide to eliminate glycerin, more decanting and last washing with water to extract the residue of the chemicals used.
Again there is a world of information about making biodiesel considering it is the fad of the moment. Thousands of people make their own fuel this way.
I decided against it because it would invalidate my home insurance, go against council regulations, I will have to pay exice to the tax man for the fuel I make and also I don't see myself collecting grease waste from my local fish and chip shop and wrestle 44 gallon drums home.
However some do it happily and here is a happy example of a farmer who makes fuel straight from the canola seed.
All in all, and considering I don't have a boat to speak of, I would forget SVO unless you have an old engine with a mechanical lineal pump and are happy to use just 10%. I would experiment with waste sump oil and a home made centrifuge. (I know of generating plants and larger boats who have a centrifuge incorporated in their fuel polishing system.)
Depending in which country you are and the differential prices and availability, heavier diesel fuel for larger ship may also be a consideration if previously cleaned from smaller particles and if in fact cheaper at all.
However all this is just my opinion not to be taken too seriously.