>People still pay for fuel?
Waste oil gets you 130,000-180,000 BTU/gal, or 10-14 kWh/L.
Veggie or motor oil, doesn't matter, the contaminants make the bigger difference.
20lbs of propane typically runs $10-$25 across the US.
Not including bottle price or deposits because that shit doesn't matter.
420,679 BTU in a 20lb bottle of propane, 17,000-42,000 BTU/dollar.
Whereas with waste oil, I currently push 30,000,000-41,500,000 BTU/dollar, as my blower pulls 30W, which means it runs about $0.00432 per hour, and I max out with this burner at 1 gallon per hour.
So about 700x-2400x cheaper than propane.
You can put together a waste oil burner for the price of a few propane refills.
You can keep your propane bottle and use it to make an oil reservoir if you're feeling cheeky, it'll hold 500,000+ BTU worth of oil at a time and it costs you absolutely nothing to fill it.
Tons of businesses are THRILLED to give away used cooking oil, they typically pay to have it removed, you taking a few gallons for free is cash in their pocket. Engine oil can go either way, many are glad to have you take it, others have a strict procedure and don't want to risk breaking from it.