Well I bought a used 40 qt turkey fryer from a guy on craigslist. That was before I realized that oil could negatively impact your brew. In order to clean it out, I took several steps:
1. Clean it out very well with the scrubby side of a sponge and dish soap
2. Boiled about 6 gallons water in it for about an hour
3. Left it overnight and still saw that there was some oil floating on top, so I kicked it into overdrive.
4. I washed it out with oxyclean, rinsed, and put more oxyclean in it to soak until I got back from classes. When I came back there was a dark black layer (the oxide layer) which was formed quickly by the oxygenating action of the cleaner, so I just thought the faster the better.
5. Finally, I rinsed out the oxyclean and then washed with bar keeper's friend, and rinsed that out.
6. I filled it with water and left it overnight and saw that there was no oil floating on it, so I assume that it's good to go.
Right now I'm boiling about 7 gallons of wort in it with no problems.
So anyway, just make sure that there is no oil in it before you brew with it and you should be golden.