Backroad,
I have squirrelled away a bunch of UMO and WTF which I plan to burn directly as diesel engine fuel by the two supply method, second supply with heaters where necessary including at the injector. Start engine on something more like straight diesel, maybe diesel and hydraulic fluid mix. I use gravity over time to take a lot of the undesireables out of the waste oil, this is to be followed up with a polishing filter that I started building a year ago and need to finish pretty soon. I am not going to worry about litmus paper strips and adding alkaline (base) to neutralize ph.
My filter is a fancy contraption that holds an 8 foot long lengh of 6 inch PVC pipe vertically with the ends capped and to be filled with kitty litter or Fuller's Earth. Oil will be forced from the bottom up through it with a small gear pump. This large pipe pivots in the center on a angle iron welded frame that allows it to be tipped to facilitate emptying and re-charging the filter medium. Pressure gauges will show when the medium needs to be replaced.
Since the first couple of feet of filter medium will be the stuff that gets plugged with contaminants quickly, I plan to bury a screened basket with pull handles so I can drag this section of filter medium out of the main pipe as frequently as needed without changing the whole load unecessarily.
I found that with WTF there are so many fines in it that will coke up the injector very quickly even with filtering through first a 10 micron followed by an element rated at 2 microns, that it is clear much finer 'polishing' is necessary on this type of used oil. Diluting the 2 micron filtered WTF with diesel did not stop the coking and engine fouling, it only reduced it linearly in time with the ratio of added diesel. i.e. 50% diesel and the thing ran twice as long before geting really sick. I was using my 1.5 Litre, 4-cylinder VW diesel engine in a power plant for this testing. I suggest that you will need to experiment if you try to use used engine oils as fuel, mixed or straight. Your results may change between batches also depending on the variability of your sources.