motor oil mixed with WVO

I just picked up an oil drum, relatively new, which had motor oil in it. Although the oil was recently pumped out and used, there is still a bit left in the drum (it hasn't been fully cleaned out). I know that diesel engines can run on motor oil, or at least mixtures of motor oil and diesel. I'm going to be using the drum as a settling tank before sending through a centrifuge for cleaning, and my question is: Do I need to fully clean out the oil drum, or is some (very small amount) of unused motor oil okay to mix with the WVO?