Adding used motor oil to fuel oil burners

When I lived in Maine I would get rid of used motor oil by dumping it in with the fuel oil tanks whenever the tanks would get filled. The resulting mix was likely never more than about 2%.
How much used motor oil can be added to fuel oil burners before they start acting up?
From what I can find it looks like they can burn around 10% and can't tell the difference and some can burn up to 30% without much problem, I'm guessing the "problems" are with the mix lighting off when everything is cold and smoke/soot.
I figure most of that difficulty would be in initial lighting of the burner in a cold box.
I could mostly get around this by using a small fuel tank, lighting the burner on straight fueloil then adding the oil as it warms up. I'll size the burner nozzle so it can run continuously, use a line heater, 2 micron filter, ect.
Anyone tried this?

 

I used to have a gun burner when I lived in NC and it ran on 90% waste oil/10% Kerosene. We poured oil in the tank through a piece of window screen and changed the filter at the burner every fall. It had a 1.0-80H nozzle. The guy that bought the place is probably still using it. No issues with soot any worse than with fuel oil, mainly because I think the waste oil burned a bit hotter. I have used it when temps were in the teens, any colder and I just didn't go out to the shop because it couldn't keep up.