If I was going to use used motor oil as fuel. Consideration of buffering the acids present in it would enter the equation. Cheap to do I think.

Just some litmus strips and a common cheap chemical. I am not aware of the strength of the acids in used motor oils but they are developed in use. Amount probably depends on how frequenty the oil was changed you are getting.

Feeding acids through an injection pump and hot injectors as a regular diet again raises questions. On the otherhand injection pumps are reasonably inexpensive at a pick and pull. I am really not an enviroment nut either. Yet what is perhaps coming out of your tailpipe might be pretty nasty.. My hope is the combustion temperature is destroying the serious pollutants rather than just converting them to even worse types of chemical emissions.

Just to be clear. I do not care what anyone uses for fuel. It is simply their choice. If used oil is going to present a lot of carbon buildup inside the combustion space. I might also consider installing a simple water injection system to periodically clean it out.

It is logical that you may benifit by moving the timing a little past the recommended advance. The flame front is probably slower on that fuel.