Ah! Now I understand (I think?).
It sounds like the original problem with turbo leakage/smoking exhaust is NOT an oil related problem at all and simply a case of bad physics (if the boost is too high and the PCV system is undersized, then crankcase pressure increases, gravity draining of the oil in the turbo is compromised and the oil takes the only other path available to it which is out through the turbo seals). Would I be right in thinking that the original problem develops as the engine gets older? As the engine's piston rings wear and lose some of their tension, and top ring groove fouls, you will see a progressive increase in blow-by past the rings. This will only partially be accommodated by the PCV system (which itself might be restricted by oil deposits) and as a consequence, crankcase pressure rises.
In this case you can forget all of that stuff I said about seal face wear and seal groove deposits!
Okay, so you've swapped out your original turbo and you no longer have smoke in your exhaust. However I'm going to guess that your pistons will be seeing a lot of blow-by relative to the original engine design. I'm going to assume that that blow-by is also seriously hot which probably means your oil is going to get hot (120 to 130°C?) on long, spirited drives. In which case, I would be less concerned about stuff like wear and more concerned about oil volatility. You really don't want a conventional oil in your car because it's Noack volatility will be way to high. I would even go so far as to say conventional Group III synthetics are too volatile. My pick for your engine might be something like Amsoil 10W30. This is a PAO based oil with a very low Noack (4.1%). Such an oil would be very oxidatively stable (good for turbos in general) and minimise the risk of oil exiting the crankcase in your blow-by (oil loss isn't the concern but piston ring fouling from burnt oil is).
You could go down the Ester route. All things being equal, these oils have the lowest Noack of all the base oil groups and they have better resistance to oxidation that PAO. However they are incredibly expensive and I'd see if you could live with PAO before considering the jump to esters.
It sounds like the original problem with turbo leakage/smoking exhaust is NOT an oil related problem at all and simply a case of bad physics (if the boost is too high and the PCV system is undersized, then crankcase pressure increases, gravity draining of the oil in the turbo is compromised and the oil takes the only other path available to it which is out through the turbo seals). Would I be right in thinking that the original problem develops as the engine gets older? As the engine's piston rings wear and lose some of their tension, and top ring groove fouls, you will see a progressive increase in blow-by past the rings. This will only partially be accommodated by the PCV system (which itself might be restricted by oil deposits) and as a consequence, crankcase pressure rises.
In this case you can forget all of that stuff I said about seal face wear and seal groove deposits!
Okay, so you've swapped out your original turbo and you no longer have smoke in your exhaust. However I'm going to guess that your pistons will be seeing a lot of blow-by relative to the original engine design. I'm going to assume that that blow-by is also seriously hot which probably means your oil is going to get hot (120 to 130°C?) on long, spirited drives. In which case, I would be less concerned about stuff like wear and more concerned about oil volatility. You really don't want a conventional oil in your car because it's Noack volatility will be way to high. I would even go so far as to say conventional Group III synthetics are too volatile. My pick for your engine might be something like Amsoil 10W30. This is a PAO based oil with a very low Noack (4.1%). Such an oil would be very oxidatively stable (good for turbos in general) and minimise the risk of oil exiting the crankcase in your blow-by (oil loss isn't the concern but piston ring fouling from burnt oil is).
You could go down the Ester route. All things being equal, these oils have the lowest Noack of all the base oil groups and they have better resistance to oxidation that PAO. However they are incredibly expensive and I'd see if you could live with PAO before considering the jump to esters.