My old man was in leather trade for 20 years he swore by saddle soap or white vinegar and linseed oil to nourish it.
I find upholstery harder as more chance of damage, people will tell you saddle soap is caustic because it's alkaline but I have never seen any adverse effects, if your a bottle snob there's tonnes of differant chems I currently have prochem two part cleaner/conditioner does the trick , it's profitable as leather sofas tend to be in nicer houses so good to tie in with carpets, repair kits for scratches are readily available pretty foolproof to get good results