I just reread this whole topic, and I figured someone better mention Dr. Jackson's Hide Rejuvenator from Tandy. It is the easiest add on sale in the world, and can new life to dried out leather. That's how you sell it. Do one small section of a customer's leather item, and they will buy the magic potion. It has saved a lot of neglected biker gear for me, and baseball gloves love that stuff. It will darken the leather slightly, but it absorbs well, without making the leather mushy. A little goes a long way. I use sheepswool to apply it, and dry sheepswool to buff it in when I am done. It will clean leather furniture on a dishtowel. (One end wet, the other one dry to buff.) I have no idea what's in it, but it smells good. It comes in a little tub, and looks like buttery Vaseline. Never put petroleum jelly on leather. Never use mineral oil (baby oil) either. There's probably a chemist here who can explain exactly why, but it's not good for the leather, and it shows.
Johanna