It's made for the leather boots of firefighters, but is widely regarded as the best leather preservative around. Even Russell recommends it, and when Russell recommends a leather goop, my friend, you know it's good.

A thimblefull will treat an entire saddle. It has beeswax and propolis (something else bees make, and a super preservative). And some other stuff, but basically we're talking about the best thing to put on nearly any leather, and certainly any Brooks saddle. We prefer it to Proofide, Brooks's own leather goop. Made in Idaho. One 4 ounce tub will treat about 90 saddles, or a few saddles and lots of boots, belts, and baseball mitts.
A new saddle doesn't need it, but after a few months, put some of this on it. Topside and underside. What the heck. Some people say NEVER treat the underside of a saddle, because then it won't breathe. If you think about it for more than a second, you'll see how little sense that makes.