Although I am not an expert in these matters, I have to agree with Jelag on the use of used motor oil in our fuel systems.
Sometimes, "things just don't sound right" & this is one of them, to me. Some of us are making extra efforts to filter our fuel to the 2 micron level, thinking that this will increase the life expectancy of our HPCR fuel delivery systems. It's hard for me to believe that anyone would put used engine oil into their fuel system.
If the old oil was good enough to put into the fuel system, why not just leave it in the crankcase? You replace the oil because it's worn out & filthy. I can't imagine putting that stuff into my fuel system.
I don't mean to offend anyone that is following this practice but, for those that are considering trying to use the used motor oil, you may want to reconsider & thoroughly investigate this practice.
Once again, no offence meant to anyone using this method & I wish you the best of luck using the used motor oil. At the very least, you are trying to introduce more lubricity to the ULSD.
Joe F. (Buffalo)