Jess used to get summer mf, well thats the vets best guess, not really sure why but I personally think its due to our sandy & flinty soil, as it drys they sink more into it and it abrades the skin and the cycle begins. The oil just makes for a mess in dry sand so I just use dry sulphur powder to heal it. I tried all sorts of creams and potions, from the vet and over the counter, picking scabs, washing with medicated shampoo and drying thoroughly, none seemed to help much. Since moving to a yard without deep sandy areas that they hang out in the problem seems to have all but gone.
Last winter jess got mf and thrush, brought silvetrasol in the end and that cleared it up. She's probably become more susceptible due to cushings and its was just the wettest nastiest winter last year, we only got 2 morning frosts here.
I just check legs quickly when picking feet out and I dont really do anything preventative in winter, other that try and make sure she's all round healthy and getting fed all she needs.