I was always taught to oil hooves as little as possible otherwise it would interfere with the moisture balance, and that healthy feet don't need oiling. So I only use oil when going to shows, otherwise I leave them au naturel - and my horse's feet have never been a problem except when he was recovering from a virus. This suggests to me that the general state of health and vitamin levels has more of an effect on the hooves than what you paint on them, so if your horse is getting good-quality feed and is healthy, why not leave the oil off for a month and see what happens?