WRX gear oil

One possibility is that some molybdenum disulfide (MOLY) was added to the oil. Similar in appearance and action to graphite, it is sometimes used where two hard pieces of metal (such as a ring and pinion geat set) slide across each other. It supplies extra extreme pressure lubrication. I don't think it is used in transmissions due to it's ability to keep the synchro rings from "gripping" the cones properly.

I found the same thing in my rear diff (also an 02 WRX wagon, actually) - I also noted what was almost like a grey gel on the bottom of the differential case (this was at 6k miles, too - I was just doing my normal "swap everything over to synthetic" treatment, rather than waiting for the gear oil to get bad...) - I was curious if some of the threadlocker which Subaru uses got in there (they appear to be pretty generous with it, and it is black in color...) - be curious to see how you like the Amsoil 2k in the transmission - I have heard people gripe that it is a little to slick for proper synchro function (no experience with it myself) - I went with Motul Gear 300 in the trans and rear diff.

As an aside, in very aggressive driving, you can overheat the rear diff, and cook the fluid a bit (which is why the STi has a temp. sensor in the rear differential) - obviously didn't happen here, but just something to watch out for.