The best way to preserve the leather is to do a search on this forum concerning leather preservation and make your own assumption based on the concerns posted.
Myself I use absolutely nothing after cleaning the holster.
Alex, lugerlou is giving you the general consensus. Some use a product called "Hide Food" if the holster is extremely dry and beginning to crack or craze. If it is in good condition, leave it alone as lugerlou says. Here is a link to a distributor for "Hide Food" http://www.autopartsworld.com. It's not cheap, but one jar lasts along time.
A community dedicated to Lugers, Central Powers, Axis, Allied and related WW-I and WW-II pistols by their scholars, collectors, owners and enthusiasts. Come join the discussion about the history, technology and extraordinary background associated with these design masterpieces.