Most Subaru vehicles with 4WD have a front and rear differential that use 75W90 gear oil, which needs to be replaced at higher kilometers.
The rear differential fluid change is fairly straight forward, one plug to drain, another plug to fill. Keep the vehicle level and fill until it spills out of the fill plug.
However the front differential is a bit difficult to get to. It has a T70 Torx plug for draining, a 8MM allen plug for checking the level; and another 8MM allen refill plug that sits on the right side (passenger side) of the differential, just behind the passenger wheel well.
I could not get the refill plug to loosen and because where it is situated I could not get any extension or good leverage to pull hard on it. I ended up refilling via the breather tube instead. The breather tube is located underneath the air duct as shown in the picture. Just fill it through there until oil comes out of the check port.