Is the factory emissions equipment still installed? Exhaust filter? Crank case vented to intake or atmosphere??
Try pulling off the lower hot side boot between the intercooler and hot side turbo pipe; if there's engine oil in there, more than a cup or 2, you might be having excessive blow by pushing oil into the intake system where it builds up and eventually gets sucked up and burned in the engine. If the truck is making oil, (either through regen or fuel leak) high oil level can cause high blow by, otherwise could be worn rings and compression blowing past the rings.
The making oil can be lessened either by removing exhaust filter and tuning, or cleaning exhaust filter. (Assuming no fuel leak internally, otherwise that would need to be fixed) Also a good CCV mod will help keep oil out of the intake.
If it's blow by from a bad cylinder, that's another problem.
Other than oil in the intercooler from excessive blowby, could be turbo seals...
Hope that helps