I've somehow taken on the role of one of Bladeforum's WD-40 apostles. I like the stuff. Its not the greatest thing ever, but its good at what it does and its cheap. In addition to propellant, it contains mineral spirits (a solvent that evaporates), and mineral oil (a light lube; same stuff that makes up Johnson's Baby Oil). There are probably (okay, certainly) better lubes (especially if you enjoy paying $11.50 plus shipping for 5 grams (link) and better rust inhibitors. But for most folks, it seems to work pretty good. Some guys say its gets gummy over time, but I've never experienced that. From time to time, I sneak around the Powernoodle Compound looking for stuff to spray with it. Leatherman expressly recommends WD-40 on its website, and that's good enough for me. But part of the fun is figuring out what works best for you. I hoard lubes just like I hoard knives, guns, MTs, etc.
It was developed to discourage corrosion. But if you are looking for lubrication (reducing wear), WD-40 has relatively poor film strength (It's mostly Stoddards' Solvent [naptha/white gas]) so far from the best choice.
I like to use wd as a cleaner then oil with a light all purpose oil.
!! Shock horror .. Sometimes i just rinse under the tap with
Warm soapy water and forget oil !!! ( proper detailed drying with a paper towel a must for this approach)
Also i only do this with basic folders that have quality ss blades ( no bearings or springs )
Mind you i am mostly useing my folders on fruit slaying missions ( soapy water quickly breaks down fruit shugars ) . Works for me , also i dont like to transfer oil taste / smell to any food prep .
Hope my 5cents helpful ?
WD-40 can turn to varnish. It's cheap and makes a pretty good solvent. It's not a very good lubricant. I used Break-Free or Tri-Flow for over 20 years and never had either fail.
Lately, I have been using Remington Oil. I bought it at Walmart. It's pretty cheap and does the job. I like it because it doesn't feel real oily.
There are much better options for corrosion resistant out there, but as a lube its alright. I mix it with some vaseline if using it as a lubricant, at first it will feel like it gunks up what ever you'r trying to lube, but after using it for a while the action will smooth out and will feel like one of the best lube ever.
Best rust preventer out there is a product called EEZOX. However it is pretty heavy duty chemical stuff and would presumably not be food safe. More of a gun thing but if you've got a carbon steel knive that you really do not want to rust, EEZOX is for you. Quite expensive and not easy to find.
Wd40 is great for flushing water from a wet knife it its pretty poor as a lubricant/protectant. There are so many better choices that you should only be using WD40 if you have nothing else. BreakFree CLP is a much better option then WD40 and its almost as cheap and easy find. Personally I use Slip2000 EWL on all my guns and knives and love it. Its a CLP like Breakfree but I like it better because its clear, non toxic, unscented and doesn't attract dust/dirt.
A knife is a very simple machine. They don't require super lubricants. WD 40 is fine. Some automatics need a better lubricant.
Any oil can varnish. WD 40 is no worse than others. WD40 is a pretty good rust protectant for items that won't be handled.
Spray on, wipe off is all you need.
I use WD40 (Water Displacement, attempt number 40) only as a cleaner to remove gunk and remains of glue from knives and other stuff.
For pure lubrication of moving parts i've never come across anything better (and longer lasting) than Nano-Oil, 10 weight for most folders and 5 weight for ones that have very tight production tolerances.