I would take Joe Dot Com's advice, which follows my experience with this type of problem. I once had heads rebuilt by professionals and they had installed poor quality valve guides. Blue smoke at start up. I would suggest that either the valve guides or the turbo seals, assuming the turbo is oil cooled of course. As the heads were refurbished, my guess would be the turbo. Makes sense after 250,000 kms on the odometer. The turbo can be refurbished at reasonable cost.
Monitor your coolant and engine oil levels. If there is a coolant leak into the engine, the oil level will rise and the coolant level will drop. The oil will become milky brown and foam will form on the underside of the oil fill cap.
If the coolant level drops only, the smoke is white and there are no cooling system leaks. The coolant has to go somewhere. I would suspect a leak into the intake or turbo (if the turbo is cooled by coolant rather than oil).
If the smoke is blue and you have no visible oil leaks, the coolant level should remain stable and the oil level will decline. Problem would be the rings and/or the valve guides. If the turbo is oil cooled, the problem could be the turbo itself or as well.
Best of luck resolving your problem.
PS: When checking for engine oil leaks, don't forget the rear engine seal.
