Granted, WD40 isn't the best for long term storage, but I've used it for a long time without any issues, for as long as I can remember to be sure, and I ain't no spring chicken!
My firearms are kept in an unheated basement where the humidity runs 75% or so for the last 24yrs, I've never had a rust problem on any firearm, leather is another issue tho, had some get moldy
, which prompted me to get a dehumidifier a couple years ago.
But if you want the best protection, even CLP is surpassed by Eezox, read his supplemental test at the bottom of the page that went on for 2 weeks between CLP and Eezox with repeated salt spray applications. I never even knew about Eezox until about 15yrs ago, my best friend started work for a mom and pop sporting goods store that sold Eezox, the owner had done a test similar to the GZ test with bright nails, the Eezox came out the winner, so I bought a can of it and was very pleased with the drying feature, the odor isn't pleasant, but ya get used to it, but it dries and doesn't collect dust like other oils. I got a chance to buy a case of it from another friend and I've been well stocked since, good stuff.
Tim