It could be the sole or the uppers. If it's the uppers, crete's suggestion would be well worth a try (though a more shoe-friendly oil might be better EG dubbin).
If it's the sole, it can be caused by the metal strip that runs from the heel to sole rubbing on its surroundings, which is far harder to cure. You need to get some lubricant into the centre of the sole unit, which you might do if you could make a couple of small holes in the sole, from inside the shoe, and drop a bit of oil in.