Check sump to see if any oil left in it !
if okay then check PVC system valves . A valve stuck shut or open will allow oil mist into the combustion chamber via the PCV system . Main valve is located under intake manifold.
Check the secondary air pump is working ( pollution control system ) it should chime in on cold start .
Take turbo intake hose off ( on four post lift ) measure turbo shaft end float and side trust with micro guage to see if in tolerance still . If not recon turbo ( BB turbos ... face book or DIY from kit on ebay )
There is a small ring ( like a piston ring ) on the hot side end of turbo shaft that wears out and is often the culpret .
The reason you are not seeing much smoke generally is because you put the cat back on and it is doing its job removing the smoke most of the time . At some point it will become over whelmed with oil and block partially . This will presurise the crancase with exhaust gas and force oil down the valve quides and past the rings .
Turbo is still my favorite for source of
problem .