Neetsfoot oil has been the traditional treatment. You should be able to find it at shoe repair stores. Sno-seal works. As does Saddle soap. Its how the products are applied, and how often that is going to assure you that they are waterproof.
There are some synthetic spray products sold to water proof boots, including suede, that you should consider using during extended outdoor use, as a " freshener" to keep the water proofing working even when the scabbard has been rained on.
Talk to your shoe repairman. He is a wealth of knowledge about taking care of leather products. Go, on-line, to the Tandy Leather Co. Catalog, and check out the products they sell. I believe you can find their Link in the Member Resources section of the index page to this forum, under " Links".
Oh, if you don't have a local shoe repairman, but live near any Amish community, go talk to the buggy makers. They are the harness shop keepers, and they also are expert on keeping leather dry and pliable. The same with Stable owners. Horses have got to be exercised, rain or shine, and you spend a considerable time taking care of the saddle, reins, and other leather gear that goes with owning horses.
In big Cities, you can find Leather Shops, where high end leather jackets, and boots, and pants, etc. are sold. They also sell products to use to keep the leather waterproof. They rely on repeat business, so its in their best interest to help their customers maintain the leather goods they buy, so they will come back and buy something else.
