Is used motor oil good for undercoating?

Used motor oil is not good for undercoating and is not recommended for car rust protection, despite old folk practice of spraying used oil under car to prevent rust. Used motor oil contains contaminants, acids, heavy metals, and sludge that can actually accelerate corrosion rather than prevent it, and it is thin, so it washes off quickly, does not provide durable protective film, and drips everywhere making mess. It is also environmentally harmful, illegal in many places to spray oil on ground as it contaminates soil and water, and can damage rubber bushings, seals, and wiring insulation under car because used oil degrades rubber. It is flammable and creates fire hazard near exhaust. Modern proper undercoating products like rubberized undercoat, oil based rust proofing like Krown or Fluid Film, and wax based coatings are formulated to creep, displace water, not damage rubber, and provide lasting protection, unlike used motor oil which is waste product. Using used motor oil may also void warranty and fail inspection. So while used motor oil may provide very temporary water repellency, it is not good undercoating, not effective long term, environmentally damaging, and can harm car components, so use proper rust proofing product instead.