Ya know, over the years I've bought and sold a few guns. I've even handled a couple more that I didn't buy.
What never ceases to amaze me is the good condition some are in after 50 years, 75, and even 100 years. They didn't have any of these wonder products back then, nor were they needed to make a gun last 100 years. Ever wonder what the big secret was?
I'll give up a few clues. First, clean the darn thing when you get done shooting it. Cleaning doesn't amount to a cursory wipe down with an already dirty old rag. We're talking about an old fashioned cleaning, using a powder solvent like Hoppe's #9, or a quality powder solvent. Then when a clean rag with some of those products remains clean when you wipe off the gun, and a patch down the bore and cylinder holes is also clean, oil the thing. Yes, any gun specific oil is good. Some might be marginally better than others, but I doubt it. My current favorites are RemOil or CLP, but those aren't magic, just oils I use. And in all fairness, I try to inspect my guns fairly regularly. That doesn't require a full cleaning, but it does include another layer of oil.
Yes, I'm bad and if I know a gun will be in storage for a while, I tend to slather a bit too much of the CLP "Collectors" on. Big deal. I can always clean it off again. Powder solvent also tends to cut it like warm butter.
Other tricks: My now deceased gun buddy, John, taught me how to oil up an engraved gun. He didn't care about the specific oil, either. His method was what was interesting. He used a soft, camel hair brush (I've heard its really squirrel hair.) He'd drip a bit more than he should on the surface, then just use the brush to smear it everywhere. The beauty of the system is that it got oil into every engraved crack, and also into places that a oily patch might miss.
It doesn't take a lot of time or skill, just interest. It does take more time than ignoring a dirty gun.
Ever wonder how guns sit in thier factory boxes for literally decades and come out looking like new? They were oiled, clean, and in a dry place.
You can improve on that just a little by bagging your now pristine and lubed up treasure. I use brand name ZipLock bags, the big ones like 1 gallon. They're thicker than economy discount house brands. Zipped shut, they tend to exclude air and moisture. Better still, bugs don't crawl into things slathered with smelly oil. They seemingly aren't interested in chewing into plastic.
If you live in a locale that has some unique features, like the gulf coast, or along a mighty river that might flood, you may need to use some common sense. I know thats hard to find these days, but look for it. Things like tupperware will hold in all kinds of food. It'll also hold out water, bacteria, mold and other things.
So you've got a smelly, oiled up gun in a ziplock bag, stuffed into an air and waterproof container. What's going to get your gun if its not a smelly, oily politician?