As glen probably found out from his google (or already knew) and some of you may know and as stated above the WD in WD-40 stands for Water Displacement so it, as glen knows from use, will prevent and break up rust (the by product of metal oxidation usually because of water) but shouldn't ever be used as a lubricant or protection.
Perfect example is moving parts....working in a bike shop ppl used it on their chains ALL the time and in the long run it does more harm then good on moving parts.
As for the OP I use camomile oil and wipe off and reapply one every month to 6 weeks