I don't know much about the turbo's for these engines mine hasn't broken yet!
But I know rebuilt turbo's run in the $700 range if that helps, better then a new bottom end but not cheap.
A while back their was a good thread on these engines and another member had a theory that the egr would drop crap into the cylinders and cause the rods to bend. You are on the same tack with the oil. However I don't think oil would do it, the engine would just burn it and if so much was dumped into just one cylinder for example that it would not, you would have a nice skip. A diesel engine will skip just like a gas engine with a bad plug, you will know if a cylinder is not firing.
While you are at it clean the intake manifold out, Oldsouth figured out how to get rid of the egr on the newer 603's. If you bolt the crossover to the manifold they will sit perfectly upside down. Then fill them with kerosene and let it sit for a day or so.