My brother-in-law's outback sport has a head gasket leak on both sides. He noticed the leak when he changed the oil last and his coolant was on low. He loses 1/3 low-to-full mark of coolant in about two weeks. Coolant and oil are both clean, and he has no other symptoms besides a bad catalytic converter. He is being told the head gasket needs to be replaced and he should just get a new motor.
I know the early phase II engines had a head gasket leak problem... subaru recommends adding coolant conditioner. His oil and coolant are both clean and his coolant system holds pressure. His car has 120k miles.