Agree, no gun oils or solvents on old leather. Over the years I have tried just about everything recommended for leather, and find that Pecards does the best job of preserving old leather.
I have a Randall knife I gave my dad some 35 or 40 years back, and the first thing he did was use Neatsfoot Oil which he used on his boots on the scabbard. The scabbard is coming apart today.
I tried Lexol on a very dry Model 1909 holster, which soaked up the Lexol like water. After several treatments the leather actually worse than before I started, but I figured several more treatments might help. The holster was a total loss after it all dried.
Mink Oil seemed to be OK, but after a few years everything treated with it formed a white coating. The white seemes to rub off with no harm, but looks bad.
I then tried Pecards, and have been very pleased with it, especially on old and dry leather. I would forego using saddle soap if it involves wetting the holster, as the old and dry leather soak it up the water like a sponge. For dirty leather I just apply a heavier than usual coat of Pecards, let it set about thirty minutes, and then carefully wipe the leather down with a soft cloth like toweling. Don't rush the treatments, letting the coats set several days before applying another. The thinner the coat the better, and don't use the sun or artificial heat to try and speed up the process.