We used to have an "old guy" that advertised "used oil undercoating".....the stuff he used was jet black, and he did my friends white dodge truck.Modern living thru chemistry sounds like a better plan. But there's no denying the old guys cars and truck were rust free so I guess old motor oil does work as well.
The "runoff" that leaked onto the outside body panels made a horrible mess, looked like tar.
His wife went ballistic, and refused to allow him to do the same to her car.
We doo have a few that use "clean/new oil" and then there is the new upstarts with FluidFilm.