After trying every lube under the sun, including greases, heavy oils, greases thinned with oil, etc., on various mechanisms, I now never use oil or grease on exposed parts. All it does is attract dirt and form a corrosive, abrasive slurry between parts. It will ruin parts faster than anything.
I only use dry films or sealing dry wax lubes on exposed mechanisms. The possible exception would be hitches and 5th wheel plates and the like, and there could still be some argument for using dry lubes there, it's just that no one has ever tried to.
On clean leaf springs I would hand burnish on some moly spray, then use Amsoil's totally unique Metal Protect HD spray. The only thing I've ever seen like Amsoil HD was hot dip chain wax which was a real pain to use. Not only that, it's really cheap considering the huge can. Since wear is not a problem on big leaf springs, I think you could still use a grease and never have a problem, though. In that case I would use "boat and trailer" waterproof grease. Sta-Lube makes some pretty gunky, sticky stuff.
Since you are probably not going to take your springs apart and clean them, I would first try some rust remover and really wash everything to get things clean. Once dry, use an anti-seize spray that has moly to condition the surfaces. Then finally some Amsoil HD, spray grease or heavy weight oil to further lube and seal things up a bit.
