Back to what Eric and I were talking about --
WD40 is ubiquitous but not well understood. As a consequence, it tends to be misused and people are often disappointed by the results. It is not for freeing bolts and parts 'frozen' due to corrosion and seizing. In that respect, it's only slightly better than plain water. (I use PB Blaster and Kroil for this purpose.)
Likewise, it only provides very limited corrosion protection. Most importantly, it is not very durable, requiring frequent re-application to ensure continued performance.
The key to understanding WD40 is in its name. It was originally developed by NASA for a specific purpose. Simply put, it was the result of the 40th attempt to develop a sprayable moisture displacement compound. So the name means "Water Displacement (formula #)40." Significant development criteria included evaporation spanning mere weeks and residue limited to specific compounds. (Most of this is well documented on Wikipedia and elsewhere, with a limited amount of 'insider' knowledge.)
Note: I've used WD40 on the special tool I bought for reaming kingpin bushings. It is not really suitable for preventing corrosion during storage but was what I had on hand at the time. Continued use for this purpose requires re-application at least quarterly. (For the actual reaming process, I used a wax thread-tapping compound.). On my to-do list is getting and using a corrosion inhibitor more suited to long term tool storage.
