I did a lot of reading - here on this forum- in the Subaru owners manual- and at oil manufacturers sites- before changing the gear oil in my '09 Foz.
The conclusion of everybody in the know- including Subaru- GL-4 will destroy the front differential.
Subaru designed the entire transaxle unit to work with the high pressure GL-5 gear oil and that is the only sane choice.
FYI- I used Valvoline Durablend semi synthetic in mine which is spec'd GL-4, GL-5, MT-1 along with the military, Mack and Ford specs . Made a measurable difference in shift feel-easier and smoother. Many on these board favor more exotic brands and blends- just try and stay away from oils with a large amount of friction modifiers designed to reduce chatter in limited slip differentials. The modifiers may make the synchronizers in the transmission less effective.
I had my car in a shop to fix a seized synchro and at the end of the whole ordeal (long story in itself) I found they put in GL-4. It was the end of the day, and I that time I'll be honest I didn't remember if I needed GL-4 or GL-5 so I drove home (about 10Km). I'm in the middle of dealing with the shop about this now so I was just wondering what the consequences would be.