You're burning oil.
Raise the vehicle while its running and check for leaks that may not show up while the car is shut off.
Check the coolant overflow for any signs of oil. That may mean you have a blown head gasket, worn rings, scored cylinder walls, worn valve guides, valve seals, and valve stems or plugged oil drain passages.
To check for blocked drain passes, take off the oil cap and have someone bring the RPM's up. If you can see the oil rising, then its guaranteed to be caused by a blocked return passage.
If thats not the problem have a mechanic do a compression test.
I listed it in this order because you can do the first test in your drive way. Most people say to do this first but most people don't have a compression tester laying around.
Edit: Also check the intake manifold and turbo compressor housing.if check the oil drain passage from the turbo housing to the engine block. If it is obstructed oil pressure will be forced past the turbo seals and into the engine through the intake tract.