Where do you store guns where the stock could get mildew? (That was a rhetorical question.) I'd be more worried about rust if the storage place was that damp.
Birchwood Casey makes some good products, but boiled linseed oil rubbed on wooden stocks has always been my choice. If it was good enough for the Wermacht and Uncle Sam, it's good enough for me. Just use it sparingly and the stock should be protected without being slippery.
A friend uses a mixture of linseed oil and beeswax on wooden tool handles. He'll scrape the paint off a new shovel or hoe handle, sand it smooth and rub that mixture into the wood. Swears it's superior to anything a factory applies to a handle. He may use it on the steel part, too.
Keep your powder dry and your guns, too.