Negative. I use LPS Industrial Strength Lubricant (#2). It displaces moisture, leaves a non-drying film and has rust-inhibiting properties. It was recommended to me many years ago by the most knowledgeable gunsmith I have ever met. It comes in an aerosol can, and is sold in most hardware stores.
Common 30-weight motor oil was recommended by the least knowledgeable gunsmith I ever met.

As I posted previously to another question about ammo malfunction, WD40 penetrates a bit too well. It is efficient at dispersing water, hence the name Water Dispersant #40. It is death to primers. True, I live in a dry climate, but even if I've had my gun out in the snow I still avoid WD40

I don't use WD-40 on any weapon I have because I have seen it do nasty things to weapons and I don't wnat that to happen to mine. I use Hopes #9 or CLP on all my weapons.

3-in-1 oil is the best gun surface wipe down I have seen yet. It IS sticky and because I hunt hard in terrible conditions I require something that is going to last through the day crawling through wet brush or tulies ... or a couple of days of it if I'm in camp. I don't use it in the mechanism. I clean the guts with Hoppes and leave it at that. When I put the guns away at end of season I will usually wipe the guts down with a light oil, sometimes WD40. The only drawback to 3-in-1 oil is the odor. For still hunters in particular this might be of some concern. I'd say at least a day or two is necessary for outgassing.

What a can of worms!

Listen up gents, go buy yourself a quart of mobil 1 0W-20. Works fantastic!

I've never used wd-40 on a firearm ever, and don't plan on it.

Bubba, if you hunt like I do (and we all know you don't), you'll need the tough stuff. Hunting from the seat of an ATV or an armchair certainly might warrant something light like WD40. It would also do fine if you're just into range shooting. But that stuff is GONE after fifteen minutes (or less!) in wet brush. 3-in-1 is still beading water on the metal two days later. My guns would be a rusty pile of junk today if I hadn't given up on WD40 thirty years ago. But for your uses I'm sure it works just fine.

USE frog Lube or CLP. I am not one to jump on bandwagons but the stuff works and is not gummy. OHH I admit that I do not elk hunt and Hunt deer from tree stands but when rabbit hunting I am in cutovers that are truly hellish and in duck swamps that are as thick and as wet as anything the west has to over. Tough Hunting conditions can be found across the country and God forbid someone does not make it harder on themselves than it really needs to be.

I have never used WD 40 on a firearm. I use Remoil post clean-up.

I use WD40 to break in a new barrel. I run a WD40 patch through before each shot after cleaning the barrel. This advice came from the VP of Engineering at Lothar Walther who makes sniper barrels for the military. Said it reduces the friction on the first 25 or 30 shots through a new barrel. It has always worked for me. I use RemOil as a lubricant and it seems to work fine.

Bubba,
My first experience with a semi-shotgun was a Remington 11-48 12ga. Every time the temperature got down below freezing the gun would go click about every 2nd or 3rd time. Lots of lucky rabbits and a few lucky pheasant. Looking back, i think it was the type of lubricants we had available in the mid '50 that were labeled 'for guns' and how we used them. Little did we know.

I use Mobil-1 synthetic motor oil. The stuff works better than anything I have found, and doesn't gum up in cold weather like other lubricants can.

WD-40 doesn't come anywhere near my guns, or anything with mechanical parts. Its great for breaking loose screws, and fine for squeeky hinges, but that is about it.

WD-40 or any other liquid in a bore not dry patched before firing risks a dangerous pressure spike if shooting max loads already.

99explorer, sulfur and phosphorous would not be additives to motor oil. These elements should typically be found in crude oil molecules (they are major elements found in the bodies of the organic materials that created the oil). But just because these "dangerous" elements are found in molecules does not mean they are going to create acids. That usually requires a significant modification to the molecules, typically an application of energy. And if there was any water remaining in the oil after distillation it would float to the top of the oil away from the gun metal when the oil is applied.

Interesting info, Jimbo. Thanks. I had forgotten about synthetic motor oil. Always wondered how that stuff was made. I always presumed that perhaps some kind of vegetable oil was the base and they worked it up from there (perhaps soybean oil?). Hmmm. Okay, synthetic motor oil, it turns out, is made from crude oil bases. Because the stuff is not refined but rather entirely remade from petrochemical derivatives (e.g. esters), these oils are described as "synthetic" (except in Germany and Japan where that labeling is forbidden). The only significant advantage to synthetic motor oil as opposed to regularly distilled motor oil is the former retains its lubricating properties at much higher and lower temperatures and also is less inclined to return combustion byproducts (soot). It would seem therefore that synthetic oil would have no advantage over regular motor oil as a gun oil substitute. I suspect that for use in harsh conditions (rain and snow), motor oil would probably perform well as a protective agent. Maybe not so well as a lubricant in the mechanism but it certainly is conceivable that one could use two different oils for gun care. And, yes, WAM, I wouldn't advocate running motor oil or other heavy lubes through the bore, not without at least pushing a tight dry rag through it a couple of times afterwards.

Similarly, when preparing a gun for use in dry, dusty environments I probably would not be using a heavy oil as a protective agent. It's not necessary and would likely only attract a build up of dusty grime. Little or no oil is needed to protect a gun in a desert so perhaps WD40 MIGHT have some utility in that particular situation. Maybe Happy Miles can shed some light on gun oils he has used in those conditions.

WAM, I did not hear about the high pressure from the guy who recommended break-in with WD-40. I haven't shot that many doing this but to me, the pressure always seemed lower with WD-40 in the barrel. Velocities dropped a bit from normal. I do use low-end charges during rifle break in and would recommend that for any new barrel where pressure is likely to be at its highest due to the new bore dimension.

No WD-40 on my guns for many years. I use Rem-Oil with Teflon almost exclusively for light cleaning, and liquid graphite for bolt action lube. Triggers are sprayed down with Rem-Oil and then re-sprayed with Birchwood Casey gun scrubber. I clean barrels with Bore-Tech Eliminator.
I might use WD-40 and a nylon pad to remove surface rust, but would carefully remove it with Rem-Oil right afterward. 40 works well on fishing reels, though.