Extend the life of your gearbox and differential, or remedy the whines and whines and notchy gear changes.

Make sure you read the entire guide first so u have a basic idea in your head before you begin.

Tools used:

5 litres of gearbox oil (technically 3.5 for gearbox and 800ml for rear diff but you might need more) I used Redline shockproof lightweight.
21mm socket (in a normal socket set, 21mm is the spark plug socket)
Some metal pipe u can stick the ratchet/breaker bar into to extend the handle
1/2 drive ratchet or breaker bar
Degreaser or silicone spray (eg rp7, nulon, wd30)
Flexible funnel with detachable hose
Oil pan
Disposable gloves (i use one incase i need a clean hand)
Jack / ramp / stands
Heaps of newspaper 

The Gearbox

Jack the car up at the front and at the back if you have enough stands/ramps.I only jacked up the front. Locate the gearbox drain plug which is located under the car roughly between the two front tyres.

For those who change their own engine oil, the screws that hold the plastic undertray is just above my hand in the above pic. With the ratchet in the pipe, its much easier to loosen the bolt. Loosen in a bit but not too loose and then take the car for a quick spin to warm up the transmission liquid. This will drain it alot better than if you did it cold and loosing the bolt makes it easier to undo when the bolt is warm since metal expands when its hot. If its too tight, spray on some silicone to loosen it up.

Undo the bolt and let it drain out. Pull out the dipstick which is located on the driver's side of the TMIC to make it drain easier. While you wait, clean the bolt and thread thoroughly using degreaser if necessary. Notice the bolt is a magnet so you might see some metal shavings stuck to the thread. If there is alot it prob means you have given the gearbox a hard time!

Once fully drained, clean the inside of the hole just to make sure there are no more metal shavings left behind then screw the bolt back on. Dont forget the washer.

Shove your flexible funnel into the dipstick hole. You might find it easier to get a light and see where you are shoving it rather than stuffing around squeezing your hand around the turbo. I bought my funnel from supercrap and its the best thing I've bought from them. The end is the perfect size for the dipstick hole!

The gearbox takes 3.5ltr according to the manual but some boxes are known to take more but remember to save 800ml for the rear diff! Check the dipstick periodically to make sure you have not overfilled. If its slightly over, dont worry because it should go down after you take it for a drive so put the car back down and take it for a quick spin around the block. Check the dipstick and make sure you have not overfilled. According to other forums, its ok to overfill the gearbox slightly.

The rear Differential

Jack the rear of the car up so its easy to access the rear diff. I assume you all know what the rear diff looks like. If not, its the big silver thing that sits in between the rear wheels with 2 big square shaped concave bolts on it. The top bolt is the fill hole and the bottom is the drain hole. My filler bolt was bloody tight and i found it easier to undo by spraying on some trusty silicone. Using the 1/2 drive ratchet / breaker bar, position your body the same as the axle so you can push against passenger side tyre using your legs if the bolts are too tight but dont push too hard! Make sure you undo the fill bolt first incase you cant get the drain hole off! There's only 800ml so it should drain pretty quickly.

While you wait, clean the two bolts the same as you did with the gearbox bolt. You will notice the bottom bolt is longer and has a magnet for the same purpose as the gearbox bolt.

Put the drain plug back on making sure its the one with the magnet. Here's the tricky bit, how do you make the oil go uphill? Unless you have a pump, you might need to use a bit of innovation because there is no way you can use a funnel due to the lack of room. For me I had 1litre bottles of oil so all i did was detach the hose from my funnel and bent it into a U shape. I filled it up and quickly shoved one end into the fill hole. The hose is small once compacted so there is enough room to lift it up so most of the oil goes into the diff. Its only 800ml so you only need to do it a few times. Fill it until it starts overflowing and just put the bolt back in once it stops dripping. Make sure both bolts are on tight enough but not so tight that the next time you do it it will be too tight.

Wipe down the diff for an spills. I had quite a bit so heaps of newspaper helps keep your garage clean.

Thats it! I really dont know how the oil affects the rear diff but i've noticed my gearbox is much much nicer to use than before. Not sure what was in there but it was clean and i assume its whatever Subaru use. The gearbox is not notchy from 1st into 2nd and 2nd into 3rd so well worth it.

Hope that helps.