Its not an sti 6 speed. It's just a 6 speed box. There is nothing special about it or STI noteworthy, they still understeer like a pig with the stock diffs. It's just a 3.9 6 speed thats a fair whack stronger than any of the 5 speeds bar the auto.
Open front diff, 4kg visc centre, R160 visc rear etc.
The good thing is they dont chew syncros like pre 06 sti boxes as there isnt a really big gap from 4th to 5th. You still dont get the good bits though, and a clutch dump in a 3.0RB in the dry will still mince a rear drive shaft (tiny).
If you ripped a 3.0RB box out and threw in a helical front lsd and 06/07 DCCD and left it with a fairly basic map the car would be transformed.
For what its worth, STI boxes range from having various front LSD's (surecrap, torsen, plated), various centres (4kg visc, 12kg visc optional on early RA, DCCD (open with up to 100% mechanical lock plates, permanant 65% rear torque bias), DCCD (06+ 3kg viscous with up to 100% mechanical lock plates, permanant 59% rear drive) 1:1 centre or 1.1:1 06 onwards, steering angle sensor 06+ etc.
As for 3.0's. They dont have oil consumption issues, they had owner issues. Buy 55k subaru and baby it around = never runs in, bores glaze over and car drinks oil. Run it on 91 unleaded and they ping their tits off = drink oil. Those that were thrashed regularly never had these issues (mine doesnt drink a drop, bought it off a bloke who seemed to like fanging the nuts off it). I'd look for one with brake dust all over the front wheels and no evidence of the engine being apart (factory gasket sealant around timing cover, rocker covers, consistent dust/dirt on everything etc) so you know its been driven properly. As a couple of subaru mechanics from then have said, every old lady and lease driven 3.0RB is a timebomb because of cheap fuel and no revs.
There is really nothing mechanical that can go wrong with them. Fresh fluids in everything as soon as you buy one and it'll feel like new. Throw it on a hoist at your local servo or mechanic and try and move everything with a crow bar. You'll find most of the hydraulic bushes (oil filled) will be rooted with wetness or dirt around them. This is normal for a 5yo 100km car driven on aussie roads. Whiteline sell a full compliment of replacements for very reasonable prices.
If the front lower control arm (rear most) bush is cactus (it will be), the whiteline replacement will self destruct without also using this:
http://whiteline.com.au/product_detail. ... mber=KBR14 due to the weird notched cut out in the factory brace on the MY05 3.0RB's. This was fixed in 2006 and in all the outbacks.
Air con compressors can die if left unused due to corrosion through the lines when people turn their AC off and drive around with the sunroof open and their flat brim cap on. A car thats always just had the AC left on will never suffer this. Not much else can possibly go wrong with it though.