Unfortunately, modern shooters have come to expect single products to do multiple things well, the result of the rampant growth and use of CLPs (cleaner-lubricant-protectant).  You know what they call a CLP?  A jack of all trades...and master of NONE.  I realize I'm going off on a rant here, but the best maintance products are those with a single use function--solvents for cleaning, lubricants for lubing, and rust preventatives for preventing rust.

We're not the military where a single product makes at least some sense for convenience, space consideration, and ease of use.

Now then, how important is a multi-hour salt spray test score?  I think we all need to determine that for ourselves.  As a private citizen who keeps most of his guns wiped down and in a safe that sports a golden rod, which circulates slightly warmed air, I certainly don't need to same level of protection as someone whose guns spend each day exposed to salt air conditions.  I like to throw a little logic into my buying decisions, as well as a little experience.  I've been using Birchwood-Casey's Sheath/Barricade, along with Shooter's Choice's Rust Prevent, for a few decades now, and have never had my guns rust because of a low score in a salt spray test. 

For what it's worth, I wouldn't use WD-40 on a gun I was selling the next day.  Evaporates too fast.  No long term value.