I use anhydrous lanolin on all my leather products (belts, holsters and rifle slings). It not only waterproofs them but keeps the leather fibers "lubricated" and supple. It tends to DARKEN leather a few shades, however, but in my case I like the darker brownish color, as it makes dry and light/pale beige (meaning usually NEW) leather look very rich. There are some tricks to applying it but otherwise, it is the ONLY thing I do to treat leather. I stopped using Neatsfoot oil and even saddlesoap years ago.
In the OP's case, since his RVN holster probably has some historical/collector's value, I would consult with a museum conservator for professional advice...it may be best to do NOTHING to it! As for newly-bought leather or not-so-historical stuff, I use the lanolin.
-- John D.