Oil in Doors and Sills
 

If I was to pour used motor oil inside the bottom of the doors and inside the sills my theory is that it would help slow down any rust that is in there.
Has any member done this?
Are there any pitfalls doing this?

Err, it will make a mess when it leaks out.

Spray a product like Bilt Hamber S50 in through the vent holes, it's designed to do what you want.

Yes I'm expecting to oil to seep out from the drain holes but after that stops do you think it'll work as a rust slowerdownera

the oil will indeed seep out, including the oil you wish will give some kind of protection (and I know it used to be done years ago by brushing on to surfaces).

The Bilt Hamber product mentioned above will be cheaper than the environmental agency fine, as well as being quicker than making a rig to clean/collect the draining oil, waiting for it to drain/dry. As well as that, the S50 should come with a 360 degree lance to make it feasible to upper edges of the sills, and the S50 doesnt risk spilling out whilst you are driving on the road - so safer for others.

Your car will stink of old engine oil!

I would use stuff that is purposely developed for protecting spaces from rust - i use Dinitrol Cavity Wax (ML) - with a 60 cm lance it can get into all the spaces - coats the surface in a film that prevents rust.;)

As an ex motorcyclist I hate the idea of a car dripping oil on the road.

Vehicles leaking diesel are bad enough - but to deliberately cause a hazard by pouring used engine oil in sills etc is just not sensible.

As others have said - far better to use a product designed to do the job without causing a hazard.

I’ve used Waxoyl before and it was easy - stick the can in a bucket of really hot water and then spray! - goes on like water, flows into all the nooks and crannies, then sets semi solid.

No leaks - no danger to other road users.

Talking of Dinitrol, I used a bundle and after two years, it appears to be holding well.

Have used a lot of rust 'eaters' over the years, and THIS stuff, that came with the KIT/BUNDLE, appeared to do a great job (it has gone up by £50 since I bought mine in 2017!!! :O:). It isnt cheap on its own though. I sprayed it inside the sills, allowed it to dry and then filled the sills with the Dinitrol wax coating.

If you have failed the MOT due to rust issues though, replacement is the only real option.

I would only use the correct stuff designed for that purpose. Engine oil is for engines, rustproofer is for cavities.

In a word No! Use a proprietory product, used car oil contails carcinogens and other potentially dangerous chemicals, a clear wax is what you need which solidifies forming a water-proof barrier, e,g waxoil which you spray into the panels.
Good luck.