Hello, I'm going to guess that the turbo's shaft bearing seal is going. Removing the turbo and fitting a rebuilt unit shouldn't be prohibitively expensive (Translation: I don't know how much it should cost). I doubt it's valve guides because she'd be smoking all the time.
Try an oil change using an oil with an ACEA rating of A3,B3. I read it's a turbo bearing additive.
An occaisionaly smoking turbo in the family doesn't anymore and a neighbors HEAVILY smoking turbo smokes truely 95% less immediately after changeover. Mobil 1 0W-30 and Castrol 0W-30 European Formula are two which are commonly available.
You could clean your intake system of any pooled oil as well. Do try one of those oils. Kira