No oils, leather dressings, soaking in water or anything else. All of these will soften the leather, it'll stretch and you'll have ruined the holster.
If you just insist on putting something on the holster to "preserve" it, contact the manufacturer and ask him what, if anything he recommends.
Most will tell you NO oils, dressing, etc.
To break in a holster that's too tight, wrap the gun in a sheet of wax paper, plastic wrap, or a plastic bag and insert the gun in the holster.
Let stand for a few hours to overnight.
If it's still too tight, add one more layer of paper or plastic and left stand longer.
Maintenance is applying a coat of a good wax.
To make the holster more comfortable to wear, the only way is to wear it.