valve stem seals, had this on my first one. leave it running up to temperature on tickover, when up to temperature rev it, if you got a load of oil clear itself out the exhaust its you stem seals. if it doesnt do it when booting it constantly whilst driving, thats because its contantly clearing it out, and its not having chance to have leaking oil build up. i'm guessing it does it if your at traffic lights for a while, or parked up with engine running, then you drive off and it dumps a load of smoke, but then clears eventually.