Blown turbo or something else?

[pugbug]

Accelerating from a stand still or hard acceleration there is a ploom of thick blue smoke from the exhaust. I take it this can only mean a blown turbo? On idle there is no smoke, the car still pulls well and the turbo doesn't sound any different. I'm not certain though, this all happened after I've been using 5W40 bottles of oil for over 4 years, but ran out and bought some 10W40.