This Wikipedia article on museum level leather conservation may be of interest. If I recall correctly, current thinking in museum circles is to not use any oils, balms, conditioners of any kind because they found that in the long term, they did more harm than good.
https://en.m.wikipedia.org/wiki/Conservation_and_restoration_of_leather_objects
I've tried neatsfoot oil a couple of times and it made the leather soft. I see plenty of people here who use it regularly and love it.