The trouble with softening old hard leather, is that once the juices work their magic, the leather is more acceptable to tears, and you will find once sitting on the seats for a time, they will tear along the existing creases of the leather. There is a product call "Leather Rejuvenate" that works really well. You wipe this on the leather seats, very generously, then cover seats with plastic bag, and tie bag, then let sit for a week or two. As I said it works really good, but the leather becomes acceptable.
Bob