In answer to your query: "Why would one oil spray the interior panel cavities. It seems resonable to expect that if a [font=verdana, geneva, lucida, 'lucida grande', arial, helvetica, sans-serif][font=verdana, geneva, lucida, 'lucida grande', arial, helvetica, sans-serif]body [/font][font=verdana, geneva, lucida, 'lucida grande', arial, helvetica, sans-serif]panel[/font][/font] was going to rust it would rust from the outside. By the time the rust spoy reached the interior oil spray it would be too late."

On my 2003 E350 van in particular, when you look into the interior / lower sidewall areas, the rust is clearly underway. If you look at the exterior of my van, there is no rust visible... yet. My experience with rust has been that it nearly always begins on the underside of the metal. By the time you see rust appearing on the painted surface, the corrosion is well underway, and is working it's way through the metal from below. The only way that I know of to arrest the process, is to keep oil on the hidden metal. Particularly in areas where roads are salted in the winter, and the vehicles are driven through salt-laced slush.
Vehicles owners from hot southern states, have a natural defence. Because it is consistently warm, condensation is less likely to form on the interior of body-cavities which cannot been seen, such as sills, interior fenders and double-walled areas. The other benefit of living in a warm climate, is that road salt is not used, and as a result it greatly reduces the electrolyitic action that occurs when wet metal is exposed to salt.