Mink Oil is the way to go, there are three types that I am aware of. Golden Mink Oil leather preserver, Mink Oil paste and liquid.
The Golden Mink Oil has additives like silicone in it and according to Fiebings was formulated for the likes of boots, the Paste is natural and the liquid is blended with Neatsfoot Oil.
I have the leather Preserver and the Paste. I use the Paste on my Barmah as much as anything else and it works a treat. As far as making leather supple it does the job very well.