Another animal product that has many uses in Lanolin.
That is good stuff for a lot of uses. You can get pure lanolin sold for cosmetic use, I have a small jar. I haven't tried it for leather, but it should be good. It is very thick and sticky, more so than Vaseline, so I don't know if you'd want to thin it with something to rub on leather. It's a natural animal product, so it makes sense it would be good for natural leather.
The specific reason I bought it though, is to protect steel from rusting in sweaty or otherwise rust-prone situations. I've used it before on steel parts of carry pistols, and the mag release on my current compact 9mm started rusting. I like to heat any steel parts up, like a little hotter than you can touch, and dip it or wipe it with lanolin. The heat bakes out any water molecules in the pores of the steel, and melts the lanolin so that it can seep into the pores better. You can wipe it off and leave an invisibly thin coating that protects and lubricates pretty well. I even read somewhere that SEALs use it on their guns and knives.