WD-40, (Water Displacing oil, formula #40) is a great moisture displacing oil. I keep several cans of it in my vehicles, and on several occasions where I've doused the ignition system, a good snort of it inside the distributor cap, and then wiped out gets me up and running very fast. If my vehicle has managed to get water in the cylinders, I pull the plugs, and shoot it in the cylinders and turn it over a few revolutions, then use it on the plugs, wipe 'em off, and reinstall, and then restart. Also use it on exposed electrical connections. It's better than a poke in the eye with a sharp stick as a penetrating oil, if there's absolutely nothing else available, and it makes a pretty good starting fluid on small engines when they're cold.
I keep a small can in my ditty bag along with a bore snake kit, especially if the gun's been exposed to rain or snow.
I do NOT use WD-40 as a primary lubricant, nor as a preservative. In a jam, it works okay for a short spell.
To the question of maintaining a Parkerized finish, I've used motor oil, transmission fluid, Ed's Red, or just about any good brand gun oil.
As mentioned, Remington 870 Express shotguns are a bead blast/blue finish, not Parked.
In a pinch, a fella COULD use a good linseed oil on a Park'd finish, but it will build up over time. Several of my 03's, A3's, Garands and Carbines have had BLO applied to the metal, and over time, it hardens, but is easily removed with acetone, bringing it back to almost it's original color. (War II guns that've been Park'd give a 'green' hue after time, but the original color can range from clear, through grey, to a deep black, depending on the metal, and application process)
Parkerized military guns and equipment that have been in storage since 1945, and had a heavy Cosmoline coating look like new when properly cleaned, and most of the long guns that went to Greece and then came back to us, were preserved in a heavy axle grease instead of "Cosmoline", and look great, and is a lot cheaper than the brand-name, Mil-Spec stuff. Just a turrible mess to clean up.
I use it on my hand tools, shovels, rakes, axes, and the like, in the fall when I put them away. Keeps the corrosion down on the metal, and preserves the wood. Helps to warm the pieces up a bit, then splooge it on with a heavy coat, let it set for awhile, then wipe it off with a rag, but be sure and dispose of the rags, or they are likely to self-ignite. I just go ahead and pitch 'em in the stove and get it over with. I do NOT use BLO in the guts of any firearm, it makes a REALLY poor lubricant! Works more like glue!
