I'm working with one right now that had hydraulic fluid in it. I rinsed it with mineral spirits, washed it out with Dove dish detergent, pressure washed it, and burned a pile of pallet wood in it. Any oil left in there is either burned up, or too scared to come out! The good thing is that many of the drums I've seen that had oil in them don't have the dreaded liner. This one was nice bright metal inside. To tell you the truth, I think it was ready after washing, but the consensus is to burn them out good. I can't imagine that a little oil residue is going to out-gas anything worse than charcoal briquets do. Burn it, build it, love it!
I'm very intelligent, but I mask it by doing stupid stuff.