There are two reasons for leather getting hard: age and dryness. Without regular care leather will get stiffer and stiffer with age. Another reason can be a shrinking due to heat. The leather in older cars was dyed vegetatively (backside of the leather is brown) and started to shrink at temperatures around 70°C. Without regular care the leather's shrinking temperature is further decreasing.
A stiff leather can be softened again to a certain extend. The shrinking process of leather, however, is permanent and irreversible. In both cases a treatment with Leather Softener is advisable, as the shrinking of nappa leather can be stopped and stiff leathers can be rejuvenated. Leather Softener is a purely synthetic oil and will neither smell nor oxidize as animal fats or oils do (e.g. fish oil or neats foot oil).