WD-40 and a wipe down every time after a gun is handled. Cheap, easy and effective, what can be wrong with that?
To be honest, I personally have not seen WD-40 gum up. Hoppes #9 will gum up and I have first hand knowledge of it.
I have read where people have used WD-40 on the outside of their guns for years and neglected the inside. When it came to a moment where they needed their gun, it jammed up due to WD-40 and other stuff gumming it up. True or not true? Personally I do not let my guns get into that state, but why take a chance? You would be surprised at how many people never tear down a gun to clean it, unless it quits functioning.
I am not using WD-40, nor would I recommend it to anyone else to use it, of course you can do what ever your heart desires.
The following reasons are why I will not use it.
1. There are far better rust preventatives out there, that are very affordable and widely distributed
2. There are better lubricates for your expensive fire arms than WD-40
3. There are cleaners that are just as good as WD-40
There are a few options beyond WD-40 that combine all three reasons above, that are affordable and easy to get. When I say affordable I mean just that, if you can afford to buy ammunition or reloading supplies for you gun/s then you can afford to pay the few pennies per cleaning more for Break Free or some other high quality product made especially for the job you want to do. Using WD-40 is better than nothing, but come on, there ARE better choices than WD-40.
I have WD-40 and use it regularly. My bother dropped one of my Ithaca Model 37's into a creek. The water was chest deep, it was in December and the water was very cold. I made him go diving for it. He came up soaked and so did the gun, and had fine sand all through it. I took it apart as far as I could and dried the parts as best I could, then doused them with WD-40 to not only get the water out, but to flush the sand out of the little crevasses and what not. I then wiped as much of the WD-40 off that I could and applied Break Free to every surface, let it dry and stored away. It never rusted, not even a little bit. The action was as smooth as ever. So I do use WD-40, but not on my guns, unless they are dropped in the water.
Remember, You are free to use anything you want, and WD-40 beats nothing, just not several other products.
Good Luck and Good Shooting