There are a few misconceptions and myths about using oil during servicing that need to be cleared up:
Oil spray under the body prevents corrosion: This is true only if rust is already present under the body or there is bare/untreated metal. In a humid marine environment like Karachi, where the air is salt laden from the ocean, salt and moisture will eventually make there way into seams and creases where rust begins. The only way to stop this is to regularly get the vehicle serviced with FRESH water (as in sweet water). Any other type of salty tasting water does indeed have salt and other contaminants in it and it will accelerate the rusting process. Even better, you should get the underbody coated with a rust preventative rock chip coating like 3M to provide an extra barrier between our corrosive environment and the vehicle.
Personally speaking, I drive an older leaf sprung vehicle that already has rust and exposed metal parts underneath. Every couple of months I get it serviced by first getting the underbody sprayed with diesel and detergent (for degreasing areas soiled by oil leaks and off-road dirt build up). This mixture is then removed with a fresh water pressure wash. No diesel is left behind on the underbody. Finally, the underbody is sprayed down with a modest amount of engine oil. Usually just enough to get decent coverage over most exposed parts, but quite liberally on the leaf springs since they tend to dry out and squeak (coil springs don't need this). Obviously, rubber parts are not directly targeted as they have nothing to gain from being oiled.
Diesel spray prevents corrosion: Diesel is too light to stick properly and it gets washed away too easily to provide any real protection. Most often, it will be used to dilute oil for spraying on the underbody as it makes it easier to spray. I don't like this because it doesn't really let the oil stick. It is also used for making the engine and other plastic parts shine, but this is about the worst thing you can do with it since it attracts dust and it attacks plastic parts like electrical sleeving, connectors, and taped joints, making them brittle and eventually useless. For the same reason, it should never be used on bumpers, mirrors and other plastic trim as it will harden and discolour the plastic and leave residue than is almost impossible to remove once it builds up.
So in a nutshell, if you're driving a newish car, no need to spray oil anywhere under it. Just get an anti-rust/rock chip coating done and put grease in all the grease fittings. Get it serviced with fresh water frequently. Oil and diesel will only hurt all the plastic under your car.
