The reason not to use any oils or most other treatments on a finished holster is because many leather treatments are intended to soften the leather. Soft leather stretches.
That's okay for gloves or saddles, but a softened holster will stretch excessively, which will ruin it.
Once a holster stretches to the point where the gun isn't retained well or is free to shift and move around in the holster, the holsters service life is over, NO MATTER how good it looks. It can look brand new and be brand new but no longer serviceable.
Applying oils or most other leather treatments to a modern CCW type holster can cause years worth of damage and prematurely "wear" the holster out.
In addition, a stretched holster that lets the gun shift around wears the guns finish much faster from the friction.
By far the safest way to break in a holster is to wear it and use it.
If it's just too tight, use the plastic or wax paper method and forget oiling or wetting the leather.
Once a holster is stretched too much by normal use or by improper fitting methods, there's no "fixing" it.
Contrary to popular belief, holster type cowhide doesn't shrink when wet, it only stretches.
You may wet and re-mold a holster to a gun but that's a very temporary fix and the holster will quickly open up even more.
If you have a holster that's just too abnormally hard for some reason, a light application of Lexol leather Conditioner, followed by just wearing the holster and gun until it breaks in should do it.
Some people complain that new holsters are not comfortable.
Jeff Cooper once said that a gun and holster isn't supposed to be comfortable, it's supposed to be comforting.