[subiesailor]

Black gear oil in the subaru? Perhaps muck finding its way into the diff via the breather or bad seal?

 

[subiesailor]

Even at 180K when I changed the gear oil after 80K run it was showing a golden brown color and still transparent - though not really the same weight as the fresh stuff going in.

 

[subiesailor]

Plain - I would also suspect more debri finding its way to the breather on the rear diff vs the front which could also contribute to this.

 

[subiesailor]

Yes, very possible.

Because the vent is just at the top of the rear cover, it's going to be in the air flow around the differential, with it's high level of road dust etc. However, in order for the dust (or whatever it is) to get inside, there would have to be a pressure difference between the interior of the differential and the outside. (The vent tube is capped.) I wonder if this would be the case. There might be some air transfer due to heating and cooling, but would this be sufficient? Perhaps.

If this is a factor, wouldn't pretty well all rear differentials exhibit noticeably darker-colored gear oil when drained, which is what steved reported? (What do you do to have golden colored oil drain out after 80k?)

Front was golden - just a few shades darker than the new stuff and a tad thinner of course. The rear I recall being darker but not by a large margin. It is a CA car which might see less water and road grime exposure via fewer extreme winter road miles etc.