For 'hard formed' leather (moulded/rigid) most makers discourage the use of oil based products... because it WILL soften the leather, and defeat the purpose of hard moulding the leather.
Instead many, like the folks at Kramer, recommend the use of something like "Kiwi Neutral Paste Wax".
Such a product will protect from water damage w/o soaking into the leather and ruining the hard molded shape (it also does not ruin the positive retention of a hard moulded holster).
Remember, for the most part it is the rigidness of a good leather holster that creates a 'positive' grip on the flashlight/firearm... if the leather becomes soft, it will not retain the item as well (think of 'floppy' kydex... wouldn't work so well huh).