An old neighbor of mine first told me of this but he would pour it directly in without filtering it. He claimed the fuel filter would clean it good enough. It was an older cummins and had over 400K miles on it but I know he had to rebuild the injector pump and replace injectors before. Not sure how often he had to or if running the used oil had anything to do with any premature wear or if it was just time for him to replace them anyways.
I think it would do more harm than good but that is just my opinion. I just don't think the price of fuel savings would offset replacing even a few injectors or the injector pump, not to mention the time it would take you to filter it real good.