I had some years old gas (w oil) in an old tank when I got my potter. The
tank was bad (rust) so I replaced it, but I just poured in a bunch of gas
stabilizer, shook it up and filter poured the old gas into my car (very
diluted with the regular car gas). No problems burning it. I have heard
the newer 2stroke oils are more advanced (meaning ashless) so are not as
much a problem to run through a 4 stroke motor (best if diluted of course
with regular gas...) If your mix in a 4 stroke is a 600:1 or something
like that on 50:1 oil, cant say I imagine that would make much of a
difference. Burn it in your car and dont worry about the "residue in the
empty tanks". My truck didnt seem to mind at all (if it even knew).