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Is it painted on the outside and bare metal on the inside? The drums seems to have all sorts of coatings and linings and some are easier to remove than others. I used a food grade drum and built a fire in it using pallets (actually two fires).
Do you know what the smoke point of the oil is? If it is a low enough smoke point you might be able to get it off just by running the UDS with all the intakes open (350-400 degrees).