The plastic bag is good advice. Milt Sparks recommends to wrap the pistol in saran wrap and do the same thing. The saran wrap might work a little better too, as it will form fit the pistol better, especially if you wrap it up a few times. Their website has more information too. Personally, I don't do anything to them at all, I just let them wear in on their own. But I don't know how tight of a fit yours is, obviously it is fairly tight or you wouldn't be on here asking questions.
Try the bag/saran wrap. If that doesn't work, you may need to call the maker (or if unavailable, another good leather maker). As far as pastes, waxes, and oils go, I'd contact the maker. One may be as good as the other, but depending on how they treated it, some may not work at all. I have never used anything other than a damp rag on my Milt Sparks holsters. They are just like the day I got them, except they fit my belt better. The retention on them is about the same as it always was --firm.
Also, maybe dumb to point out but sometimes things get missed and you didn't mention exactly which holster you have, but some have a retention screw on the side. This will tighten or loosen the holster. Another given, wear the right gunbelt, belt loops, etc. If you wear the holster sloppy and it is a well made holster, the pistol can get stuck.
And no, the open top is not retention. I can still walk up behind you and relieve you of your pistol in short order. I feel this type of holster is only good for concealed carry (in which you are concealed --enough so that strangers can't tell you are carrying) or for range use or anywhere a surprise disarming isn't likely to happen.
I see all the open carry protests here and think how irresponsible a lot of them are. One of these days, someone will relieve one of them of their pistols and start a mess. Whose fault do you think that will be in the civil case? If you open carry, use something with a thumbreak or something like the Blackhawk Serpa, which has the retention on the trigger guard (when you draw, your finger should be over this area anyway --au naturale!). Otherwise, keep your non-retention holsters at the range, home, or concealed in public.