Not an expert but the last thing I'd do is soften a holster. I've used neatsfoot oil, etc on ball gloves but not on holsters.
Depending on detail of the boning and as to fit, I've used the thin plastic bags found in grocery produce section and used it to form fit the holster. Place gun in bag and then into holster after some wetting either with water or alcohol based treatment to mold the leather, not soften it.
I've tried the more expensive hand-boned holsters that required some breakin. Since I don't care for a thumb break or top strap but prefer the less expensive holster with a tension adjustment (usually a screw).
Virtually no breakin required and the only treatment is care and conditioning of the leather.