Forgot to mention in my reply above that I use an air compressor to blow out all my holsters after a few months of use. Keeps them cleaner and less likely to scratch up the guns finish. You would be amazed at what comes out of them!
IMHO NO softening substance or "soaps" should ever be used on a leather holster. The idea is to keep the shape, retention qualities and stiffness as long as possible. That way your gun will be secure, positioned properly and be less likely to do damage to the finish.
After many many years of carrying guns in Cowhide holsters that had snaps, screws, straps, linings, etc. I discovered Horsehide Holsters. Horsehide holsters (I buy mine from Kramer Leather) are thinner, lighter tougher, smoother, and hold their shapes and retention qualities LOTS longer than Cowhide does. I also find that with the Kramer Horsehide there is absolutely no need for extra retention devises like the snaps, straps, screws, etc. and kept clean like I mentioned above will minimize scratching on the guns finish.