Prior to electronically controlled diesels this was not an uncommon practice. With the tighter tolerances associated with modern injection systems, I wouldn't consider ANY oil, used or new added to my fuel. But then, injectors and pumps are not cheap for my Cummins ISX. The OEM's definitely recommend against doing it. And at $400+ for a new injector, let alone all six, I am not going to throw oil, especially used oil, into the tanks. Of course, Cummins would recommend it's Centinal system.... they would get to sell more injectors!!!
To each his own.....