Start with the basics first,so you don't overlook anything.
It could be oil,or it could be fuel.
Check the air and fuel filters first.A plugged air or fuel filter could do it.
Start by inspecting for excessive blow-by,pumping oil up past the CCV,and into the engine.Also check the turbo,to make sure it isn't pumping oil into the intake.Removing the turbo to intake boot will allow you to inspect for oil.
Check compression,belt timing,pump timing,ect,to make sure nothing is off.
If all that checks out,then you may have an injector or pump problem.