Neatsfoot oil, same that I use on my King Ranch saddles. They have a non darkening formula if you don't want the leather to darken. I do. The darkening also lasts longer between applications as well, probable is why it darkens the leather over time.

Lexol would be another option, but a little higher then just Neatsfoot. I would check your local saddleshop, farmer's Co-Op, or TSC and see what they have.