I am actually in the midst of trying to make a decision on whether to switch to synthetic or not. My previous car was an 1997 Audi A4 1.8T Q and was told to use synthetic because Audi/VW won't warranty turbos that didn't have synthetic oil in the engine.
Now, Subaru is not saying anything about synthetic for turbo, etc. The service manager at local dealer says older mileage cars can convert to synthetic, which is contrary to the leakage theory you mentioned.
I've also been told that once you switch you shouldn't keep going back and forth...I'm so confused. Can anyone provide some objective, more data driven information on this topic?