My experience is that whilst it quickly washes off from wheel arches, old oil is an excellent preservative of bits of the underbody that are out of direct spray from the wheels.
It's also good inside box sections, as the high petrol (etc) content allows it to creep well into seams.
I also have found that it helps to preserve home-applied oil-based paint coatings by slowing the rate at which they dry out and start to crack and flake off.
Its creeping abilities make it useful for spraying partially hidden areas of the chassis, of which there are a few that a paintbrush or spraygun can't directly reach on a Wartburg.
The best plan is to rustproof one year with Waxoyl or similar, then the next with old oil, which keeps the Waxoyl supple and sticky. Alternate like that and you won't go far wrong.
Disadvantages: Messy, smelly and dirty; need protective clothing and a good mask. Not really suitable for immaculate cars. Smell takes some time to fade; drips of dirty oil on driveway after application.
Advantages: Cheap, effective and easy to apply. Ideal for rustproofing a winter hack or a car that will never be concours, but is worth preserving.