Yeah, burning waste oil can be interesting, unlike using propane or a fuel oil, your fuel changes every time you add more waste oil to the barrel. The waste oil seems to separate a bit over time with water on the bottom, then the heaver oil and the lighter oils towards the top.

On my Barrel I put a "T" fitting where I draw the oil from. One side goes to the heater and the other side has a shutoff valve that I can use to drain out any water that might have connected over the summer.

.