Years ago I worked as a photocopier/office machine repair tech. We tried using WD40 as a lubricant for the old manual typewriters. WD40 at that time actually said to use it as a typewriter lubricant right on the can. Yada yada yada our company agreed to an out of court settlement for damages and the WD40 can doesn't say it can be used on typewriters anymore.

Like others have said WD40 is more of a solvent than a lubricant. It will wash away any grease or oil present and leave a rock hard varnish behind that is next to impossible to remove. I remember as a kid I found this out the hard way. I used WD40 to lubricate the brake, clutch and throttle cables on a motorcycle. They all froze up solid after a couple of weeks. I could even pull the cable out of the housing when it was clamped in a vise.

Great for loosening rusted fasteners but other than that I don't use the stuff now.