It turns the leather dark and it pretty much stays that way, like mink oil. I don't know the other reasons but collectors hate it. Something about destruction of fibers. I won't even use it on work boots after discovering Obenaufs. I use the 'paste' and the liquid, depending on application. The liquid for jackets, etc., and the paste for thicker leathers. BTW, here's the holster I mentioned. I treated this one heavily inside and out because it was so dry it was starting to crack. Now it's still properly firm, but supple. I could have cleaned it with naptha - you can see the dark places on it, but did not want to remove any 'patina' - that's how confident I was in the Obenaufs.
As for shooting the pistol, get a new 16# recoil spring and extra-power firing pin spring, sold in sets by Wolff springs. Most would also recommend a Shok-Buf too. This is to reduce battering of the slide when it hits the frame.