Don't just use protectants, cover your entire winch if possible. There are PVC covers available, but even a simple plate that deflects rain water away from it goes a long ways (it can also make a nice platform for stepping on while working on the back of the engine for example). Humidity aside, "heavy" water like from raindrops (or dunking the winch in creek) is your biggest enemy as it likes to collect in places where it takes a while to evaporate, if you can minimize exposure to that both the cable and the entire winch will live a longer and happier life.