There are various preparations on the market presented to address that issue. I have used some of them, but not any more. It is inevitable that wood will dry out. Storage in an environment with 40-60% relative humidity will help preserve it. Many conservators will recommend lower, but if your are keeping your firearms in your living or working space, it needs to also be comfortable for you. Hopefully, It was sufficiently dried before it was made into a gun stock in order to avoid self destruction due to drying, shrinking, and cracking. You can't reverse this process; only repair the resulting damage, to some extent.
If no repairs are required, I would clean (I usually use cotton tipped swabs) lightly with Ballistol (mostly medical grade parafin), and protect with Renaissance Wax. Be careful with the Ballistol around brass or silver. It will very quickly remove the oxidation layer (patina). I will just use Ballistol on non-ferrous metals to remove some of the dirt and patina if it is completely obscuring what is beneath, as in engraving. All conservation efforts should be done deliberately and with a light touch.
For working guns with an oil finish, I would, occasionally, apply another coat of finish. I would NOT do this with a gun made in 1840.
I hope this helps.
