Regarding the carbon on the shaft that you observed, it is indeed highly likely that the rear shaft seal was leaking 10 reasons a turbo leaks oil (and then smokes) turbochargers are a fantastic way to add huge power to any powerplant, however, if the system is not designed properly you can spend a lot of time and money repairing turbos and chasing issues When the rear seal leaks, it allows hot exhaust gases from the turbine housing to enter the bearing housing
Bad Turbo Seal Symptoms at Laura Painter blog
This contamination can cause the oil to degrade rapidly, leading to sludging and coking
To put it short, the seals themselves rarely fail and the oil leaking from the turbocharger is just a symptom of another problem that is waiting to be fixed
Here are some of the issues that could cause your turbo to leak oil: Oil is very often overlooked as a critical component Turbo / turbocharger oil leaks can be caused by a variety of factors which often originate around the correct pressure within the compressor and turbine housing. Another way to identify the oil seal leaking is by letting the engine idle for a min or two and then pull away normally (engine under load) if seals are leaking there will be some blue/white smoke, which then clears
This should tell you if the exhaust side oil seal is knackered.
