You can use the same oil in diff as well as gearbox on the STI and wrx but it's would be better if using it spiritedly to use a nice LSD oil that's a little thicker like a CRX LS 75w/140 millers. I'm guessing this is what Ryan has from Alyn as its what I get from him for the diff
I use redline lightweight shockproof in the gearbox itself which is not cheap! but supposed to be decent.