If it can be sucked out of the tank, I suppose the oil company could take it back.
If it's been used, I presume the neighbours could rely on an estoppel argument.
I think the interesting thing would be if the neighbours said that you could, in theory, take back your oil ... but not our oil that was already in the tank. given that the two oils will have mixed, it is thus impossible to take back the oil company's oil. If the oil company don't have a contract with the neighbours, then there is no applicable "retention of title of mixed goods" clause ... so I think the neighbours would get to keep the oil.
Just thinking as I type, here.