good question. i recall reading that petroleum based products can break leather down over time. Wax based products like sno-seal don't but can attract dirt and clog pores (but i still like it :thumbup). I have a pair of nubuck hikers and treated with this stuff last year.
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nikwax seems to be a good name. it's liquid and soaks in readily. seems ok but haven't used it long enough to  :thumbup or :thumbdown.  Maybe someone here can add.