It’s a well-known fact that dehydration is as bad for the unfinished fingerboard of your guitar as it is for you. You get thirsty. Your frets get thirsty. It happens eventually.
The way to prevent frets from drying out is to periodically oil them.The natural oils in unfinished rosewood, ebony and pau ferro fingerboards can dry out. That’s bad. Stop that from happening and keep your frets in great shape by giving them a drink—by periodically hydrating your fingerboard with lemon oil or other non silicone-based wood/furniture oil.Use lemon oil or other non silicone-based wood/furniture oil and apply with a lint-free cloth. Then rub it in. Then let it soak. Then wipe off the excess. That’s all you need to do.
And remember—no oil on finished surfaces and no oil on maple fingerboards (since maple fingerboards are, um, finished surfaces). Fender by Meguiar’s Fretboard Conditioner is an excellent product for keeping the fingerboard of your guitar properly hydrated. You may want to pick up a polish cloth as well.