https://www.youtube.com/results?search_query=waste+oil+heater
Some folks may build a used oil furnace
The best ‘used oil’ furnace is a wood stove with a small hole drilled in top for a small stainless steel tube with a flow-control valve.
The ‘used oil/waste oil’ is stored in a barrel overhead and to the side and runs through the small (1/8”-3/16”) SS tubing and into the firebox.
The flow-control valve only allows a slow drip of oil.
You fill your wood stove with good dry firewood and let the oil drip for awhile.
Then you light the stove as you normally would.
If you have the oil control adjusted right the firewood won’t burn and only serves as a wick for the oil.
Keep the oil barrel full and you can have a wood fire that never goes out or needs filling with wood.