It won't hurt anything to use WD40, but there are much better products like the ones that have been mentioned in this thread. All I can tell you is I have never seen a knife manufacture recommend using WD40 and a lot of the products mention in this thread are highly recommended by the manufactures. WD40 is a thicker lube and has been known to gum up a bit. The Teflon based lubes are much thinner and with teflon it's a great moisture dispersant.
QFT