This is the exact same problem that happed with the VW Passat 1.8T sludge issues. Ultimately VW recommended only certain synthetic oils, a larger oil filter, and a 5000 mile OCI. I ended up going with an even larger filter used in the TDI engines just for the added oil capacity and filter element size.
The VW issue can be summed up like this:
Small oil sump capicity + small filter + long standard OCI + conventional oil + hot turbo = coked up oil sludge.
I don't have a Legacy GT (I have the 2.5i) , but if I did own a turbo charged car I would use nothing but a quality synthetic and would not go over a 5000 mile OCI without a UOA, and would use the largest oil filter that would fit on the car.