The reason Castrol has "turbo tested" on their protec is because its about the cheapest oil that is good for use with turbos.
Castrols synthetic line also are tested with turbos - read the label on the back. All brands will recommend synthetic oils for high performance japanese engines, and although I dont have 40 years experience, I'd tend to believe what the oil companies recommend. Its also interesting that the Japanese performance companies, eg blitz, trust etc only offer synthetic oils.
In regards to it being to thin, my understanding is that synthetic oil was developed to help with the problem of mineral oil getting to thin and not handling high temp/load - so if anything at high temp it should be giving much superior protection.
I notice my car manual states gt4's were delivered from factory with mobil 1 and they warn against using mineral oils.... interesting
Turbo rebuilders suggesting mineral oils might be reusing bearings and seals and don't want the oil leaking out at idle.