Sounds like oil passing the rings. A leak down test will help pinpoint.
A constricted/blocked turbo drain will cause similar symptoms, but I doubt it's the turbo as they generally smoke more under boost if they seals are leaky (btw seals rarely fail, it's generally another fault such as blocked return or worn bearings that cause seals to leak)
You can also remove the oil filler cap and place your hand over it, get some body to give the engine a rev. If you feel pressure under your hand as the revs blip it's likely crankcase overpressure causing the smoke. Which is generally rings or can be a blocked crankcase ventilation system. If you get an indication that pressure is building in the crankcase get the ventilation system checked and a leak down done.