Under vehicle oil wash... question

[68gmsee]

I was talking to an old timer (sounds funny being as I'm considered one these days) about rust under my trucks and he was saying that back in his heyday they would do an oil wash on the underside of vehicles to prevent rust. Rather than dismantling, he seemed to think that was the way to go.

Any of you had this done? Maybe shed some light on what is involved or how it's done or is it worth looking into?