[DrD]

Well, the maintenance schedule would just have you inspect it and has no replacement interval out through 132months/132k miles. In past Subaru's I have owned (all turbocharged and MT) I'd replace the gear oil in the transmission and rear diff every 30k miles or so, and that is certainly my intent for the front and rear differential for my current car.

Nothing wrong with preventive maintenance - gear oil (even the fancy pants stuff - I'm partial to Motul gear 300, myself) is cheap.