I agree that the heat of a turbo, gas or diesel, is over-rated as far as general use goes. There are always execptions such as race or highly-modified engines, but overall, turbo's aren't really that hard on oil.
The reason is that the exposure to the heat of a turbo is a very brief instant in the total cycle time of the oil. If you were to imagine one ounce of oil going through the total lube system, then it's exposure to the turbo becomes very infrequent. It's not unlike passing you hand over a lit candle. If you HOLD it there, you'll burn, but if you just gently pass over it, even somewhat slowly, you'll disipate the heat with the moving mass of your hand. Same thing happens with the oil. That's why I said that a PROPERLY operating engine with a functional lube and coolant system will likely NEVER see the "exteme" heat that synthetic oil manufuacturers want to warn us about.
The only time I consider turbo temp to be an issue is extended heat soaking upon shutdown. If you get the turbo really hot from a hard pull and then shut it off immediately, the turbo could scorch the oil. But the way around this is to always drive a short bit of "cool down" time, or allow the engine to idle a few minutes before shutting it down. This is only necessary if you really heat up the turbo; normal around town driving from day to day does NOT greatly heat the turbo. The best way to track this is with the use of a pyrometer. Generally, 1400 deg F is too hot during operation for extended periods of time. 900-1000 deg F is considered a good operating temp for a diesel, and idling a diesel can come down the less the 400 deg F in a very quick amount of time.