Griot's Garage Leather Restoring Balm.
It is like honey... you have to rub it in gently and let the case sit up or hang where it can get air for two weeks, to dry. Then do a second coat. After that dries, you can treat it with Lexol, which does not turn slick when wet in the rain.
I have treated saddles and scabbards from the American War Between the States, rolled up slings for Mausers from 1918, USGI 1911 holsters from WWI, and brought them back to where i can use them.
I am getting ready to restore a leather case and bellows on a 1937 Zeiss Ikon camera. I will take photos before and after.
If the leather is very dry or molded, you may need to treat it first with a thin solution used by libraries to kill microbes on leather books. Then you treat it with a gentle restorer. I have some of both, but the labels are stained, so I will have to read them and where to buy it.