I wouldn't fancy running them on used oil. When they say multi fuel they mean diesel or kerosene. I use kerosene which is a lot cheaper than diesel.The "cab heater" type Chinese burners have really gone up in price since the energy/cost of living crises. They use a heat exchange so they don't generate CO2 into the building. Even on diesel they work out much cheaper than leccy heaters, and they are multi fuel so I suspect people are burning other fuels such as used engine oil.