Mulder

It's 3.7 qts. Info is in your owners manual.
The manual also specifies that only GL-5 rated gear oil should be used, MT-90 is GL-4 rated. Yes people have been using it but technically it does not meet the spec indicated by Subaru.
Redline 75W90NS is GL-5 and can be used in the trans.

Mulder

It's not a durability rating, it indicates the fluid meets industry standards for the application and is compatible with the materials in the unit (in this case trans) in which it will be used. Using the wrong fluid and changing it more often will not make this "ok".
Go to Redline's site and read their info on GL ratings.

DrD

The GL rating can be loosely interpreted as the ability of the lubricant to function in extreme conditions - the higher the rating, the more effective the gear oil is at dealing with extreme pressures. (the API spec's for GL-4, GL-5, etc. quantitatively define what the oil must be able to do) Subaru specifies a GL-5 rated oil because, in their judgement, the transmission and/or front differential and the rear differential generate pressures at mating surfaces which are such that a GL-5 rated oil is required to get the job done.

The difference between a GL-4 and GL-5 rated oil is (when speaking of oils from the same manufacturer) only really in the concentration of extreme pressure additives which are used - the GL-5 having two to three times (or more) as much.

It's never a good idea to use something which is of a lower performance spec than what the manufacturer recommends... MTL or MT90 might help with synchro function, but may not be able to get the job done for the gears in the transmission and differentials...

Mulder

Shockproof isn't GL rated and isn't recommended for street use, although as with MT-90 there are people using it. I have asked Redline about it in the past and they advised against using shockproof in street driven vehicles.
GL-5 is what Subaru recommends and if you want to second-guess them, fine, it's your car and your decision- but get all the facts first.
As DrD said it's possible that oils other than the recommended type may make the transmission shift more smoothly, but this doesn't mean they will provide the proper protection against long-term wear of the gears and bearings.
There's actually nothing wrong with continuing to use organic gear oil of the same kind that is in the car from the factory, it will provide plenty of protection.
Transmission wear and possible damage has as much, if not more, to do with your driving style and skill level, and level of mods done to your car, as your choice of gear oil.
I'm personally using Redline 75W90NS in the trans, and Redline 75W90 in the rear.

Mulder

I changed mainly for the protective benefit of synthetic and easier shifting in cold weather. I had previously had good experience with Redline in another car, so I decided to try it in this one.
I changed at about 3500 miles, not because it was needed for maintenance but for the above reasons.
I find that the shifting action is smoother with synthetic, and noise in the lower gears was noticeably reduced.
I didn't have any grinding or other issues with the factory fluid.
First gear does not synchronize well while moving and is best engaged at a stop with a second or two delay after depressing the clutch, but this is par for the course and I don't attribute it to the fluid. This is more or less normal for these gearboxes regardless of what oil is used. Some have reported improved ability to get into first by using different fluids but as we discussed above this may be a compromise in other areas.
All other gears are fine. I am now approaching 45K on the car.

Mulder

Yes it's best to change the fluid warm so it flows out better. Also be sure you have the right tools and equipment, a wrench for the drain plug and a long transmission funnel to refill (it's done through the dipstick).
As for mixing it isn't a problem, synthetic and organic fluids/oils are fully compatible with each other. You do want to drain out as much of the old fluid as possible to ensure a complete change but if a bit remains behind it won't do any harm.
No, you absolutely do NOT add friction modifier to the NS when using it in the trans. That's the whole point. The difference between the NS and standard Redline 75W90 is the NS has no modifiers, the regular does. Friction modifiers are desirable in "wet" type LSDs and also have benefit for bearings and gears, but are bad for manual transmissions because they interfere with the friction needed for the synchros to operate. The NS is the proper fluid to use in the trans for this reason.
It is actually perfectly ok to use NS in the rear diff as well. I chose to use the 75W90 with modifiers, as per Redline's recommendation.