Wrothgar, . . . two words my friend, . . . no oil. Nada, nein, nyet, no, none.
ANY oil will have a detrimental effect on your holster if it is made of vegetable tanned leather, or any part of it is (and I have never seen one that was not made with veggie tanned leather).
The vegetable tanning process produces a leather that will take and hold a shape when it is wetted and shaped. Other leathers have a different process and can be oiled. Oiling a veggie tanned leather will cause the fibers to swell up a bit, and push away from each other. Eventually, instead of having a nice, firm, molded, solid leather holster, sheath, scabbard, belt, or other item, . . . you will wind up with a diaper like floppy, glob of fibrous nothing.
Some holster makers will put a very light coat of neatsfoot oil on the outside of a leather item before sealing and finishing the object. That oil is good for the life of the object, . . . however many years that happens to become.
Baseball gloves are supposed to be soft and supple, . . . like driving gloves, . . . holsters are not, . . .
It's your holster and your money, . . .
May God bless,
Dwight