First thing I do with any new leather holster I intend to use, is drop it in a boiling pan of water for 2 or 3 minutes. Then I Vasoline the gun I intend to carry in it, and put it in the holster, and let it air dry. This takes the slack out of the holster, and kind of custom fits it to the gun.
I then take the holster and give a good dose of plain old saddle soap that I use on the saddles for my horses. This gives the leather a good water resistant quality, and will take any squeak out of the leather.
I'm sure that boiling leather probably isn't real good for it, but with proper care ( and having good leather in the holster to start with) , they will last for 20 or so years.
P.S. Be sure to clean the gun and re-oil afterwards.