[chipmunk]

For whatever it is worth.... I haven't read previous posts, so if someone already mentioned this... oh well!
GL3 & GL4 oils do not have as much additives as an GL5. The theory is that the additives build up a protective sacrificial layer on ferrous materials to prevent wear & tear. The more pressure and temperature, the better the ZDDP film build up. However, this protective layer can be harder than the yellow metals. So in an event of extreme shock, the protective layer will take away the yellow metals with it and cause pitting. That's why GL5 is not recommended for synchro transmissions.

BUT... I emailed Subaru customer service, and they said that as long as it's Subaru transmission oil, it should be fine. I don't expect much expertise from a customer service guy, so I didn't nag him too him.

Anyway, back to your car....

You picked a car that has almost 60k miles. Who knows what the previous owner did/didn't with maintenance. If the dealership put GL5 recently, it won't be long enough for it to be the cause. It is most likely the previous owner.
And yes, it does sound like a synchro issue. That does not mean that the synchro tips are rounded off or blunt, it could also mean it's not getting enough friction. If there is no physical damage, then a simple oil change will fix it. But if there is (not to sound negative), hopefully the dealership might help you out since you just bought I from them.