Oil spray is about the best thing for rust prevention. And also yes get it dirty after you get the oil on. Rock dust or just road dust works good. Nothing muddy, you want dry dusty dirt of some sort to embed into the oil. get enough on there and it it starts to turn into a nice hard gray crust that repels salt and water. Its ugly and messy when you need to work on the truck but having a frame not rusting to dust and chunks is a good thing in exchange.
i think after i find a way of getting the rust off this is what i will do
This last week end I was replacing my bed on my truck. Before I put the newer bed on to the truck I quite literally painted (couldn't find a cheap sprayer =>) the entire box on the bottom side including up inside the fenders with oil prior to putting it on the truck.