I add oil to my undercarrige every day.
I don't recomend it.
As for the actual question at hand.. I wouldn't do it to something of value. An old beater/crap bucket that you are just trying to get a few more years out of, yeah sure. But something of value... I wouldn't bother.
The difference between today, and 50+ years ago, we travel down the freeway in the rain at 70 MPH all the time. The blast of water will work the oil off of there. And, looking at it from the 'green' point of view... every bit that gets washed off, has to go somewhere. One quart of oil contaminates 100,000 gallons of drinking water.