Not sure if it's the worst, but I heard of guys using recycled oil? I just don't know about that.
At the local pick-and-pull here, they sell used oil in gallon containers. None of it's labeled and could have come from any car, might be any grade, have who knows how much life left.. What a gamble. Yet I can't get through the checkout without seeing at least a couple people in front of me purchase a gallon or two. IMO, that would be the worst oil to use, though I have no experience trying it. Maybe they know something I don't!