Sorry, but you've already lost the battle. Your problem is the rust that is already present. You'll never get rid of it. Anything you do will only seal it in. And every time you throw up a rock off that gravel drive, or that piece of salt you kicked up out on the highway, it will start more rust. Not to mention that nice salt bath you have to drive in all winter will find every possible crevice. A film of oil applied 6 months ago (or 6 days ago) won't stop anything.
I always Ziebarted my vehicles when I lived up north. It works but only if applied from the git go. I still remember driving a new car straight from the dealer to the Ziebart place - they still insisted on steam cleaning it before the application.
Don't the domestic manufacturers all still have a factory 100,000 mile warranty against rust ?
And for the record, I really don't want to follow anyone on a rainy day who has sprayed any kind of petroleum product on the chassis of their truck. I imagine all the motorcyclists are especially thrilled at that idea. Yes, I used to ride (that's how I got the camping bug).
Lived in Iowa for 48 years so I do have some experience. The state bird is made out of salt.
Tim