Not on the '05 onwards STi. The filler is hexagonal with a (very expensive) temperature probe in the centre, don't break it removing it!

Unless it's pissing out oil, the level will be fine if it's been serviced at City's.
Mechanical diffs clunk, it's what they do. The noise is the reason Subaru didn't give us the good rear diffs until 2005, then quickly changed over to helicals.

Slightly off topic, but it seems to me there is a bit of a misconception on what the DCCD actually achieves, or I don't understand it properly. People are always saying that you can move the power to the front or the back by adjusting the DCCD in manual mode. My understanding of DCCD is you are only adjusting the amount of slip between the speed of front and rear diff, so you can go from fully locked to fully unlocked when you are adjusting it.

Back on topic, I have found that you will get the clunking in auto in a carpark on tight turns, but if I put the DCCD in manual fully unlocked(Green), I can completely get rid of the clunking. Full lock is when you will get the greatest amount of speed differential between the front and rear diff, so by unlocking the DCCD you are allowing it to slip. If you were to put it on lock, it would force the front and rear wheels to travel at the same speed and cause the car to buck and make all sorts of unpleasant noises.

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GOLD PIG STI

Same experience. I had a full lock turn into my garage on fairly coarse bricks. If it wasn't in manual fully unlocked you could hear the rear tyres fighting to not slip and the rear diff clunking as they would finally give.

Have a gander at some of the drift cars with a locked diff try chuck a tight turn ( not spinning up the rears ) and your AWD system is basically doing the same thing. Just unlock it for tight turns and it will reduce stress on things. It won't break it but why make it wear more than it has to.

If you put a heavier oil in ( I ran Smurf blood for a while in the diff ) and it wouldn't let the diff slip as much. Surprise Surprise the clunking was worse.

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Stoner & Webber 2011 World Champions