The parameters considered here are cost, ease of application, effectiveness, and life. Used motor oil coating will need application once or twice a year but it's free and surely it will not last more than a few thousand miles.
I don't like thick, permanent or semi-permanent coating. In my previous work experience, such coating cracked, chipped, or peeled. It makes moisture trapped between metal and coating, making corrosion process much faster and worse. On my previous truck, it was undercoated by unknown source and the coating was damaged; so salt-ladden moisture was retained.