IMO, it was a defect in the design of the oil system (or maybe the oil recommendations). This site has some info as well as the TSB's from SoA:
http://store.forcedperformance.net/merchant2/merchant.mvc?Screen=Info_SubaruOil
And because of this, Amsoil doesn't even recommend exceeding any of Subaru's intervals on any Subaru turbo.
Quote:
Subaru has published Service Bulletin # 02-103-07 that identifies a factory design related problem with premature clogging of the oil mesh screen located inside the oiling system that supplies the turbo charger on all model turbo charged cars. A clogged screen will result in oil starvation and turbo charger failure. Subaru has since published Service Bulletin #02-110-10R indicating that 2010 MY and prior turbocharged engines continue to require oil and filter change intervals of 3,750 miles (6,000 km) or 3 3/4 months, while all 2011 MY turbocharged engines have returned to the original requirement of 7,500 miles (12,000 km) or 7 1/2 months, and they are required to use synthetic oil. Due to the issues outlined in this document, AMSOIL recommends following Subaru drain interval recommendations.
Here are details of the 3,750 mile interval:
Quote:
Due to heat generated by the turbocharger and carbon deposits contained in exhaust gas, the oil in a turbocharged engine will deteriorate faster than the oil in a naturally aspirated engine. Therefore, special care should be taken to use the proper grade oil and to monitor oil deterioration.
Under normal driving conditions, the recommended oil and oil filter change interval for turbo vehicles is every 3,750 miles or four months, whichever comes first.
However, for vehicles driven in conditions beyond normal, such as racing conditions, the oil and oil filter may require more frequent changes.
http://drive2.subaru.com/Sum08/Sum08_Turbo.htm
This is all before Subaru recommended synthetic. And there were lots of cars that had turbo failures that were dealer serviced at 3-3,750 mile intervals.
-Dennis