Originally Posted by Linctex
Used motor oil in very small percentages would be fine, but the pump and atomizer designed to use No.2 heating oil will NOT handle used engine oil in large percentages because it is just way too thick.
I would imagine it would also clog the nozzle and burn dirty in a hurry. Not to mention it'll clog up the tank filter (and pump screen if equipped, I know my Beckett burner does) quicker, over time they get dirty with just N2 heating oil. I'm still cleaning up the mess the previous owners of the place I'm in...