After having used oil analysis' done on a WRX and FXT, I wouldn't run a conventional oil in a turbo unless I stuck to a 3,750 oil change interval.
I used M1 10W30 on the WRX and M1 5W30 in the FXT. Five mile trips weekdays, and a few trips including SC in July (100F days with the FXT). A little bit of off-roading in the FXT, spirited driving in both but no track/auto-x in either.
The 10W30 helped up better (as expected) than the 5W30, both at a little less than 6,000 miles. GTX is very good stuff, but I wouldn't go less than a 10W30 in the summer and I would use a 10W40 in temps of around 100F.
Run the GTX for 3.5-4,000 miles then send in an oil sample, IMHO. Top off what you sampled and wait for the results.
http://www.blackstone-labs.com/free_test_kit.html
That way, there's no guessing how long you can run the oil.
-Dennis