I have had the best success with lard, the lard one can buy in the grocery store.
A riding teacher, who I lesson with once a year or so, one day told me that they had been cleaning out the dusty corners of their barns and found a stash of old dried up jumping saddles. She asked me if I knew of ANYTHING that would bring those saddles back to usability.
I told her about the lard. Well the next year when I had my yearly lesson she told me that the lard had worked well, the only down side was it was messy getting the lard into the leather. But when she saw the results it was fine, they saved the saddles and could use them again. She gave me my lesson for free in thanks for helping her save the saddles.
WARNING–lard melts rubber big time. Keep it off of any rubber like material, if you get some on rubber clean it off immediately.
WARNING–lard corrodes chrome covered metal, turning it into a corrosive green substance that eats leather. Again, if you get any lard on metal clean it off immediately.
Otherwise it works super well. When I lard leather thoroughly, working it into the leather with my fingers, it leaves the leather soft and supple after several applications (I do three applications at first, then use the leather on a horse so the lard will soak in better, and I re-apply the lard maybe three more times in the first year.
When I do not use the leather it does not dry out anywhere near as much as it does without the lard, and it is super easy to bring back the leather when I work the lard in a few times.
Every once in a while my main riding teacher finds an old bridle that had been lost years ago and forgotten. Every time she asks me to do my leather magic with the lard. I do get the bridles usable again if there are no cracks in the leather.
Lard is cheaper than the made up leather care products, and a tub of lard can last me for years.