Having carried rifles in scabbards for close to 45 years now, the only thing I do is a light coating of Mobil 1 Zero to-30 on the rifle.
It is not the rust that will get you, it is the wear itself of metal on leather. The more oil you have on the leather,
especially inside, the more dirt/grit it will collect where you can't get at it. Some guys like fleece lined scabbard, but that collects moisture and you will definitely get rust from that.
Rifles carried in scabbards get scabbard wear, sides of scope bells and muzzle. Same as handguns carried in holsters a lot.
Pretty much fact of life if you hunt on horses or mules and I don't think any oil is going to stop that.
With a particular 1936 Winchester Model 94, I put it in one of those gun socks then into the scabbard.