OK, what he said, rust proofing oil....
We do a lot of beach work here in Oz.... Major point is do not use ANY detergent oil, the detergent is an OXIDIZER that will promote rust... If you are going to use engine oil, use ONLY NON-DETERGENT...
Here in Oz for salt water we use specific rust proofing oils, Fish Oil, and other rust proofing materials with phosphoric acid base which eats rust...
Just don't use an detergent motoroil, which is almost all of them, as you will DOUBLE THE RUST....
Best Regards from the Ranger "On the Beach" beat....