Palm Oil
The best oil for preserving Bible leather is found in the human palm, not sold commercially.
Fred W. Cropp, president of American Bible Society, received a letter asking a question: "What do you recommend for keeping the leather on the back of Bibles from getting stiff, cracking and peeling? The reply was, "There is one oil that is especially good for treatment of leather on Bibles. In fact it will insure your Bible will stay in good condition. It is not sold, but may be found in the palm of the human hand."