Depends on what type of leather is on a boot; suede, split leather, hard uncoated. Stokdgs idea of using Red Wing Oil on Red Wing boots seems logical and great choice, in my opinion.
I use Sno-Seal by Atsko, Inc in Orangeburg, SC 29115 for all my boots and work shoes. Best waterproofing and salt-blocking leather protectant I found. Not meant for suede or split leathers, however.
For patent leather (the shiny, black shoes worn by the military or wedding grooms) I use Kiwi`s Parade Dress Black shoe polish and a wool pad shoe buffing block. A little saliva spit does not hurt, either (looks disgusting, though, to spit on perfectly good shoes).
The only place I`ve used car leather protectant (Griots Garage Leather Care, the cream that smells like a barnyard) on is leather-bound and covered books and Bibles and leather-covered chairs. Works really good on the books and Bibles who`s leather has dried out, especially softer calf or cowhide leather of older Bibles (not so much the bonded leather on most books and some Bibles)