This is a good discussion with multi pronged approaches and varied results. Frequently case straps and handle bases, parts that take strain, are affected the worst. In my restorations of 100-150 year gun cases I’ve found that some leather dries out to the consistency of cardboard, it has no strength, shreds easily. If not bent sharply or pulled or strained it will hold it’s shape but is only useful for static display. Call this type 1. The other type I frequently encounter is very hard and inflexible, it cracks easily if bent. It will sometimes continue to serve if not badly cracked. I call this type 2. Type 1 can be made to look good with most commercial non liquid leather dressings but nothing will restore the lost strength. I haven’t found anything, including Neat’s Foot Oil that will restore the suppleness to type 2 items. Any actual experience on items like this would be appreciated.