Best guess is the factory oil was conventional. You can use synthetic of the proper grade with not issues. Some folks feel that you should use conventional until you are certain that the motor is sufficiently run in. This would be indicated by oil consumption either stabilizing or (better yet) being non existent. I switched to Mobil 1 5/30 at 1100 miles with absolutely not issues or any measurable consumption. Good Luck.
i looked at the oil chart at the service dept when i was there and it lists the '12 outback as using conventional oil.
so, i suspect your '12 and my '12 came from the factory with conventional oil.
at my last oil change the tech performing my service switched me to synthetic... i know this because he left the unsed portion of the oil bottle wrapped tightly in a clear plastic bag in my passenger footwell