I would try a more robust oil after you get the problem fixed. Amsoil does not even recommend running the SSO for more than 3,750 mile intervals. I've run it to 8k and had so so UOA results and have seen an STI run at around 7,500 miles and it was only "ok" as well. Although, if you're running a bypass set up, have removed your AVCS/turbo banjo bolt screens, and doing uoa's with very good results then you're probably fine.
From Amsoil's product application guide.
2005 SUBARU LEGACY 2.5L 4-cyl Engine Code EJ255 Turbo
LUBRICANTS & FLUIDS:
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.
I would run the Amsoil HDD 5W-30, DEO 5W-40, or maybe
the ATM 10W-30 at 5-6k intervals.