Well, just noticed the other day when I was stuck in traffic. The car heated up a bit more than usual (fan came on). I then drove away from a junction leaving a cloud of light blue smoke. I know this from common sense to be oil, but is it the valve stem seals or the piston rings? I have been given the impression that piston rings give smoke on acceleration, while the valve stem seals give smoke on the overrun. It only happens when the engine gets to normal operating temp or above and I blip the throttle. If I move off very gently it gives off hardly any smoke.