Here's a board for oil stuff, and they have WRX and WRX STi used oil analysis data too:
http://theoildrop.server101.com/ubb/ultimatebb.php

Here's a long-term test of two very popular synthetic oils in a large V8 motor with data every 1,000 miles, and extending beyond one's wildest mileage dreams:
http://neptune.spacebears.com/cars/s.../oil-life.html

Here's a dated FAQ (old but nobody's getting younger and it's a start):
http://www.micapeak.com/info/oiled.html

The board in particular (first link) is a great site and almost everything anyone ever wanted to know is there, so it's worth searching their archives. Also check out the Subaru data, it's really interesting.