Can't argue with that, but dealing with oily residue is easier then dealing with crusty rusty residue.

I had a truck that I bought cheap ($1200) that was fairly clean underneath. I mostly drove it through the winter in the salt and slush, then to make matters worse, through most of the summers when it wasn't being used as a truck to haul stuff, it got parked on the grass along my driveway. In a matter of 4 years, the entire bottom of the truck was a dark brown rust color. The rust was bad enough in those 4 short years that nothing bolted together under the truck came apart with out breaking the bolts. I felt fortunate that nothing serious under the truck needed done before the frame rotted out bad enough (at about 6 years) that I junked the truck. From up above the truck looked pretty good, but it wasn't. Not sure an annual oiling would have help that one much, but I could see where it might be helpful with my 49.