Hi Jay! I don’t find old leather cases useful at all but keep them to give away if I sell the cameras. I clean with water or saddle soap if water alone seemed insufficient. If the case is a finished leather and the finish is relatively intact I’ve applied a hard wax, like Renaissance wax, and buffed it out with a horsehair shoe brush. Once I renewed a finish with shoe polish and it seemed to work. I’ve never seen a benefit of neetsfoot or mink oil on a camera case.
I've always used saddle soap and a bit of warm water when cleaning leather camera cases, as Dan recommends. Once dry, follow up with Johnson paste wax which is a hard finish wax which doesn't feel greasy or oily once the solvent has evaporated and it's been buffed in.
I feel any kind of oil applied risks transferring it to the lens or viewfinder windows when the camera is put in the case and handled.
I generally save leather conditioner products for leather straps and parts that should remain pliable.
Otherwise it's polish/wax on the outside of the case.
Since most of my cases are clean, yet need some regular maintenance, I use neatsfoot oil with some regularity. Quite inexpensive if bought by the gallon at a tac shop. For cleaning I use Allen Edmonds shoe cleaner only because I have it on hand.
The utility of an old case depends upon the original quality of the leather as well as how it has been stored and cared for over the years, so there can really be no general rule based upon age. I have several Leitz Benser cases bought in late 1960s and early 1970s that are as good as new except for straps.
With many older cases, it’s not the leather but the stitching that must be replaced. Restitching can be tedious and time consuming, but often worth the trouble.
You should lightly wipe down the leather with a damp cloth (avoid soap) or a little bit of white vinegar if they are very grubby. Then allow it to dry and apply a good quality natural balm with a soft cloth. Let it dry and soak in, then buff off! Oh and only apply to the smooth side of the leather. I make and sell a really good one made from beeswax, sweet almond oil and cocoa butter.
If you care for your leather goods they will last a very long time!
As previously mentioned, it is very important to check vintage leather for signs of wear. A strap is only as strong as its weakest link!