I think the rumor started because of the huge amounts of detergent present in ATF.
There are detergent boosters available over the counter, but the only "snake oil" I would add is Marvel Mystery- it really does do things for lifters and sticky valves. But, if there isn't a problem, I don't add anything but high quality oil to my crankcase.
Any damage done by the gasoline isn't going to be reversed by anything you can add, and any gasoline that was in there is gone.
Besides, Willys engines are tough, I'm sure its fine.
