When I retired about 35 years ago, I cleaned up my leathers with soap & water, then gave them a good dose of the magic elixar, Coacholine, came in a dark red tin, workes great. Dug them out 3 years ago, still supple & in good nick. Trouble is, I'm 10kg heavier, squeezed into them for 1 meeting, then hung them back up again. Still have some of the old stuff in the tin, but I don't know if you can get it know. Probably lots of other options, check with your nearest shoe repairer.
Mike