US Marine,
You first ask "What lubricant?" and then go on to tell us that you use WD-40 for when your gun gets wet.
I won't pretend to be the fount of all knowledge on this issue - there are probably dozens of chemical engineers and metallurgists on THR who can tell you the science behind this better than I, and many who have handled guns far longer than I - but I will tell you the following:
1) Lubrication is different from corrosion protection.
2) Lubrication is simply a means of making sure that there's no metal-to-metal contact between 2 devices, or 2 pieces of the same device.
3) Using something to wipe down your gun when it is wet SEEMS to have little to do with lubrication, and lots to do with corrosion protection.
4) WD-40 is OK for both lubing and protecting, but ONLY for a short time (i.e. as an expedient measure). WD is a chemical that was invented to whisk water away from the surface of Atlas ICBMs in the 1950s (see, the time spent waiting for repairs in a service station isn't always a complete waste). It therefore HELPS to protect from corrosion a little bit, since we're talking about iron-based guns which rust when in contact with water. It merely displaces the water (
Water
Displacement, Formula 40), it doesn't place a barrier between the metal and the moisture. WD also gums up after a time. Again, it is OK to use as a temporary expedient - and it'll also help to clean the gun a bit (but so will water, and you don't really want to use that).
If I HAD to use WD, I'd degrease my gun and do a thorough cleaning, lubing and protection job on it ASAP afterwards. I used to use it a lot, and I found that it gummed before long, plus didn't really protect against rust too well. Use it for your garage door, not your guns (and grease is probably better for that).
5) Some people prefer to use a CLP (cleaner, lubricant, protectant), since using 1 bottle/jug is a heckuva lot easier and less expensive than 3. Among these, Breakfree CLP is among the best, and so is FP-10. There are others, my object is to merely mention a couple without starting a pissing contest. The drawback is that the jack-of-all-trades is a master of none - in real life and in gun care. Either of the 2 will do a pretty decent job in all 3 areas, but by necessity they cannot do as good of a job as 3 separate, dedicated chemicals - the dedicated chemicals are far closer to 100% devoted to the particular task at hand (less, because of propellants, stabilizers, etc.), whereas the CLPs have far less than 100% of the formula dedicated to each of the 3 tasks.
6) Good lubricants for guns can include Lubriplate (mostly white lithium grease - and buying large tubes of the stuff in an auto parts store is dirt cheap vs. the branded product) for use on older weapons with not so tight tolerances (like Garands and M1As); Mobil 1 20-50 V-Twin (for a relatively cheap by volume and easily found substance) and various dedicated oils and greases (Slide-Glide, Tetra Grease, Rem Oil, etc.). I just visited a thread regarding Mobil 1 (and other motor oils) that is REALLY interesting reading, though long:
http://www.thehighroad.org/showthread.php?t=291494&highlight=mobil+redline
7) Good corrosion inhibitors are far more necessary for longer-term storage, and probably make sense as a usual preventative. Eezox is generally recognized as the best, though many will dispute that. You want something that will not displace water, but actually form a barrier against the penetration of water. In theory, with that stuff on a gun you could submerge it into water and the gun would be protected for quite a while. The VERY best corrosion protection is multiple layers - keeping your gun in a humidity controlled house, in a safe with a Golden Rod and dessicants, with an excellent vapor-barrier product and coated with Eezox gives you nearly perfect protection, for example - any of them alone would be highly effective, but multiple layers bring your chance of a rusted gun down to way under 1/2% over a long period of time, if you do your job.
What do I use?
1) CLP - I used Breakfree for a long time, and have switched to FP-10.
2) Cleaner - Some foaming agent. Spray in, let sit, clean out. Much less work, though more time. Against copper deposits, pretty much any ammonia-based copper remover will do the trick, but be very, very careful about where the stuff goes - it'll ruin gun finishes, and can ruin your barrel if you leave it in for too long. You can also use automatic transmission fluid (a pint for like $2, try THAT with any gun-specific cleaner) for most cleaning and do a pretty good job.
3) Lube - Lubriplate on my Garand, M1A and milsurps. Lubriplate on the rails of my semi-auto handguns, with FP-10 as a field expedient (its always with me when I shoot). I just bought a pint of Mobil 1 20W-50 V-Twin for this purpose, and I'll put a little at a time into a squeeze bottle with a small tip for cleaner application. That article in #6, above, really convinced me.
4) Corrosion protection - Rig from a Rig-rag for my carry guns (applies in seconds over the entire outside of the gun, used EVERY time I unholster it); Corrosion-X in a spray bottle for most other guns. When it runs out, I'm getting some Eezox - many articles and tests I've seen have convinced me it is the best stuff.
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That all being said, the best thing that one can do for ANY gun is to take care of it, which is to say: clean and lube the sucker fairly often, and particularly after each use. I have a few milsurps, one older than any of my grandparents, and I'm pretty sure that none of the fancy chemicals that have been discussed on this thread even existed when they were built and used (and several were probably used HARD, in war). Taking care of a gun well and often is the best preventative and lubricant...though doing so with better chemicals makes for a longer and more trouble-free life for them.
