Your car's PCV valve is an inexpensive but often overlooked component. Long ago, we would simply vent crankcase pressure into the atmosphere -- nowadays, we vent it into the positive crankcase ventilation (PCV) system instead. The PCV valve helps to control this ventilation and also helps to regulate air flow.
A clogged PCV valve can cause increased oil consumption, which is bad, especially in Subarus. :)
Remove the old PCV valve using a 3/4" socket and extension. Ensure that no particulate can fall into the opening before you can install the new one.
It's possible to inspect the existing PCV valve prior to replacing it, but I find it's easier to simply replace it since the part is so inexpensive. If you'd like to inspect it instead, check to make sure that the ball/plunger at the end of the valve can freely move; it should make a clicking sound when you shake it.
This is the exact PCV valve you'll need.
Using a torque wrench is highly recommended as it ensures the PCV valve will be tightened to the proper tightness. Torque wrenches are an inexpensive but essential tool in any DIYer's automotive toolbox.
Set your torque wrench to 14 lbf. ft (or pound-foot) and tighten the PCV valve until your torque wrench clicks.