Don't use oil, grease, lard, mink oil, bear fat or other liquids on leather unless you intend to have a nice soft leather sock.
I don't know why it would squeak, I haven't seen that in years. Is the squeak coming from the belt and holster rubbing? Try some wax or shoe polish on the offending areas.
If the holster is generating the sound by itself, try working some talcum powder into the seams and see if that helps.
Galco sells a product called Leather Lotion and that may help but I can't guarantee it.
Honestly, I can remember the last time I dealt with a squeaky holster and I know who made it but that was a lot of years ago.