From some observation a full synthetic is essential for a turbo motor, specifically for the turbo itself and particularly if the turbo is not water cooled.
For example, a friend of mine has run a RB30ET engine with the crappy original bush bearing turbo using Mobil 1 for more than 40,000 hard klm, including a lot of track work, without touching the turbo or the engine. I've changed over to Shell Helix full synthetic, simply because Shell products seem to work well and are moderately priced. We'll see.