Are you letting the motor idle several seconds before shutdown? This is necessary to allow the turbo to cool and avoid oil coking in the hot center section. Even with that precaution, the oil cokes some, which is why you change the oil religiously at 2.5-3k mi on a turbo. You also have much higher combustion pressures, which causes more blow-by past the rings, further contaminating the oil. Like a diesel, in other words, and diesels also require more frequent oil changes. I've run Castrol 20w-50 dino for eons, in turbo and n/a motors, and I swear by it. I ran straight 50w in the hot rod Fiat spider (12:1 compression), but that was overkill.

I might send out a sample for peace of mind, but I think you're fine.