Snow Seal
This is what I have used for 40 years:
http://www.atsko.com/products/waterproofing/sno-seal.html
Sno-Seal is a natural beeswax waterproofing which protects leather from rain, sun, snow, and salt. It dries to a solid wax that "stays put" in the surface of the leather so the protective film lasts longer. Greases, oil, and animal products permeate through the leather and clog the pores, filling natural spaces that are supposed to allow perspiration to pass through and insulate.
In my experience use of animal fats weakens and rots leather. Tanneries work hard to remove fat to preserve the leather, so it's hard to imagine why you'd want to put it back on.
Before waterproofing is applied, the boot must be clean and dry. Heavy dirt is scrubbed off with a wet brush. Light deposits can be wiped away with a cloth moistened in water, mineral spirits or Stoddard solvent. Solvent has the advantage of evaporating quickly so you don't have to wait before applying the waterproofing.