This is good advice.The statement below is from the guy I've gotten all of my leather holsters from.
"Your holster will require very little in the way of
maintenance. Keep it clean and dry. Do NOT oil it.
If it should become excessively dirty, use a slightly
damp rag to clean it and then let it air dry. ( No heat ).
As it ages, you will see normal wear and scuffing
occur. You can apply Fiebing's Leather Balm
occasionally to restore that like new shine and give it
a wax protection."
http://www.jeffreycustomleather.com/ColorsAndOptions.html
For what it's worth, I've never oiled my holsters intentionally. I just let the oil from the gun transfer itself to the holster, and just wiped down the outside when needed with whatever cloth was handy.