My most humble opinion: The boots already have a layer of protectant on them just by the fact (assumed) they have been worn. I would just use a good ol' horse hair brush with just good ol' water and brush until they are clean. After that put on the protectant of choice. When I was in the military I knew guys that could make a pair of boots last seemingly forever using just good ol' Kiwi(TM) shoe polish. They had to get them resoled long before the leather even looked "worn".

I am not a representative either paid or otherwise for KIWI products.