Yeah, oil tanks are pretty stout. Consider that many of them spend decades outside or underground ... and only a few rust & leak. Most are at least 1/4" thick steel.
When the wifey & I got married in our yard several years back, my buddy brought his oil tank spit. His was just an oil tank cut in half with a bicycle chain & sprocket setup connected to an electric motor to turn the rotisserie. A couple pieces of square stock welded to the bottom formed the support for that. Could handle a 500 lb. pig if there were such a thing.