I remember covering undersides of cars in the 1980's with old engine oil.
As a precautionary note the general advice back then was to avoid getting oil on any rubber bushes.
I stongly advise AGAINST used oil to be used as an anti- rust treatment.
As used oil contians a lot of acid formed during the combustion process, and microscopic metal parts, it will do more harm than good...besides, it will be VERY messy underneath the car...
Use dedicated products, use it the way it's supposed to be used.
If someone insists in using oil as an anti-rust treatment, use NEW oil, not used, reasons see above.
But it will have little results, as it will wash away very soon during driving in rain etc.
No photos I'm afraid, but I've just seen a 59 plate, 72k example in a slightly worse state. Perhaps more important than the flaky surface rust on the spring cups was the hole in the corner where the tubular suspension arm is welded to the cross member. Certainly an area that should be examined closely.
I've also seen a 10 plate with similar mileage on which the suspension was remarkably in a much better condition.
The spring pans crack about 25mm inboard of the welds to the swing arm. Soma also rust through the bottom.
Waxoyl thinned with white spirit is very effective. But thinned chain saw oil is amazingly good. It soaks into the metal and really seals out the weather. It's also biodegradable so any drips on a drive will eventually weather away.
Over time it sets to a sticky varnish so don't leave it on paint work.
If you axle is really too far gone you can replace it with a Fiat 500 axle and springs. Use the top rubbers at both ends of the springs and you will get a better handling car with a smoother ride.
The mod probably does not work on a 100HP due to larger wheels.