Besides the ATF, I have also heard of using WD-40 and chainsaw bar and chain oil. The bar and chain oil is sticky and apparently also contains additional "corrosion inhibitors" beyond the oil itself. This would be shot on with an undercoating gun. I couldn't take the mess of either of these methods (although the ATF residue might not be too bad). Each fall, I purchase a case of Eastwood Rust Encapsualtor (big spray cans) and spray the entire underside of the truck with it. Yeah, it costs about $150 bucks for the case, but is easy to apply and with a "straw" on the can nozzle, I can get it into some tight places. I first power wash the truck thoroughly, then spray it with a cleaner like "purple power" and then power wash again. Then spray the rust encapsulator. Is all this a big PITA to have to do every year? Yes! But it keeps the underside in decent shape and (with proper cleaning) the paint can be touched up or reapplied at any time. Plus, you don't have to deal with that oil film when working on the underside of the truck (well, except for the oil that the engine leaks anyway) As far as in the doors, up under the wheel arches, inside cab corners, etc, spray those with a good rust proofing (the oil/wax type) that sticks but will also flow into the seams before it "sets up". No matter what you do, driving the truck in the winter salt is going to cause some rust - no getting around it. If you want it to last as along as possible, do not drive it in winter. Unfortunately, that is not an option for me - winter is when I really need the truck. And regular under body washes do really help!