It depends in part how the holster is finished originally. For example: Any of the high sheen lacquer types will seal the pores of the leather and won't let it accept any wax or cream - really a low-maintenance finish, but also one of the less natural finishes out there. Non-lacquered, traditional paste luster leathers will only soften with oils, so only waxes and glycerin should be used, and then only sparingly, if ever.
As a leatherworker with a degree in chemical engineering, I've never been a fan of Renaissance Wax for any "using" leather gear. It's a microcrystalline wax, which makes it fantastically stable and enduring for preserving museum pieces, but that structure also means it can damage the collagen fibers in the leather as the leather flexes and abrades itself against the microcrystals. The net result is weakening of the leather over time, apparently softening first, but largely breaking down the "hand" of the leather.
Largely, holsters shouldn't need anything. I use Fiebings paste wax for most of my holster work, only when they absolutely need it - which is almost never.
Galco EZ Draw can be used to help break in the holster to your pistol, as can simple rubbing alcohol - but care must be taken to be understand the nature of the finish, as alcohol can strip or streak colors (and can re-establish some of the firmness lost if a holster is over-oiled). Soaking in water and inserting the pistol wrapped in a ziploc bag then drying will improve the fit as well, but it also runs the risk of poor drying and introduction of mold.