I know this thread is a little old, but I'm going to share what I learned last year just in case you havent had the best results or someone in the future is wondering the same thing and comes across this post.
I have a 20+yr old pair of leather boots, for 20 years I where them almost all winter long and until last year I didnt apply any kind of conditioner or anything. Usually I would give them a good scrub in the sink and then apply a black shoe polish a few times per year. So after 20+ years of wearing and storing them in the dry high dessert of Idaho, the leather was VERY dried out and generally stiff.
Last year I started doing research on both new and traditional methods of making leather boots water proof since my feet always got wet in the rain. I did a lot of reading and, to make a long story short, I came to the conclusion that the traditional methods are the best for leather. This method involves saturating the leather with natural oils that block water absorption through the pores in the leather hide. For about 4 days I applied a home made paste of coconut oil, cocoa butter, beef tallow, and parafin wax(next time I will use bees wax, much better for the application). I would basically set my boots on the heater vent after rubbing in the paste, then let them sit about 24hrs. After 24hrs they would look like nothing happened, still dry looking. But after about 4 or 5 days, the leather had absorbed so much that it started to look like damp conditioned leather again.
Now, what this has to do with softening leather. After multiple applications of the homemade cream, the leather became not only water proof but very soft and supple. Also, I learned that leather is very similar to our own skin. When it gets dried out, it can take multiple applications and a little time to soak things up. The rule of thumb I learned is that if it is something you would put on your own skin, it will be good for leather. So things like; coconut oil, cocoa butter, shae butter, heck even just regular lotion will work. Just dont use oils or fats that have a tendency to go rancid over time (for example olive oil and bacon grease).
Anyway, hope this helps someone out. Good luck!