We weren't talking about a catch-all Lube, but a rust prentative for storage purposes.
Camu has pointed me the way toward something better to use than WD-40. Its expensive but excellent in rust prevention and its also an EP lubricant. Its called "Slip 2000 EWL." Quite possibly the best gun oil ever made.
For storage and rust prevention it has the advantage over WD-40 in that you don't have to leave a heavy film in place that will collect dust. And since its a supreme lubricant all you'd have to do is check and re-lube when its time to go shooting. With WD-40 you should wipe it down/remove it and lubricate with a good lubricant.
This Slip 2000 does it all. I stand corrected.
But WD-40 remains a cheap, available and excellent STORAGE oil to prevent rust.