On the TTRS the bevel box hangs off of the back of the transmission casing and is the power take-off to the Haldex at the rear axle. On the mk1 TT with a similar arrangement the transmission and bevel box shared fluid, on the RS they are completely separate housings.
Here are the stated capacities and part #s for oil. The bevel box and differential use the same type but the gearbox is different. I bought 3 one liter bottles for the gearbox and 2 one liter bottles for the bevel box/differential.
Standard disclaimer. Transmissions are very expensive. Take it to a mechanic if you're not sure of what you're doing.
Transmission oil: G 052 171 A2
Rear diff and bevel box: G 052 145 S2
Transmission: 2.4 liters
Bevel box: 0.9 liters
Rear diff: 1.0 liters
Tools required:
Some sort of fluid pump
5mm allen socket
10mm allen socket
extensions and wrench
M8 triple square, T25 torx, T30 torx (for the undertray)
The procedure for changing the oils is to fill until the fluid runs back out of the fill hole. In order to get the proper amount of fluid in before it overflows the car needs to be as close to level as possible. So get the car securely in the air and level, whether that's on ramps, jack stands or a lift (for the lucky few). Another helpful tip, Always check to make sure you can locate and open the fill plug before opening the drain plug. You don't want to be stuck with the fluid all drained and no way to refill :)
Start by removing the front undertray to get access to the transmission.
The fill plug is on the front of the transmission casing just above the aluminum support. You can see the hose coming out of the intercooler in the background of the picture. The fill plug takes a 10mm allen socket and conveniently has a taper in the seat so no need for a crush washer and you'll know when it's tight.
The drain plug is easy to locate. Just follow the seam down around the bottom and you'll find another 10mm allen plug. Audi was nice enough to use the same fill and drain plugs so it's ok if you swap them. That's the dogbone at left to help orient you.
The drain is a large diameter hole and the fluid comes out fast! It's 2.4 liters worth so be sure you have a large enough pan to catch it all. And it smells pretty bad too.
Once it's drained close up the drain hole and use the pump to slowly fill the 2.4 liters back thru the fill hole until it just starts to seep back out.
Next move around to the bevel box and make sure to switch to the proper oil. You can find the drain and fill holes near the passenger side half shaft. The fill plug is hard to see so start by locating the drain plug. That's the half shaft at the top of the picture and the oil pan and level sensor to the right.
The fill hole is located on the same housing but up higher and tucked back in. Here the long allen socket is in the bolt, you just can't quite see it. The drain hole is bottom center.
This is the same procedure as the gearbox. Drain the housing, close it back up and slowly pump in just about the full contents of a one liter bottle.
Finally move back to the rear axle for the rear differential. NOTE! There are two housings here, the Haldex and the rear differential. These instructions are for servicing the rear differential only, the Haldex uses a completely different fluid and mixing them up will cause damage.
The rear differential is further back and sits behind the swaybar. In this picture the black bar running under the housing is the swaybar, the back of the car is to the right and you can just make out the Haldex controller at center left. This is another 5mm allen socket shown here in the fill plug. The drain plug is also a 5mm allen and is located just over the swaybar, just right of the shadow from the wrench. Same procedure as the other two but this one should take a full 1 liter bottle.
Clean up any drips and double check for leaks before putting the front undertray on and putting the car back on the ground. Take it easy for a bit to make sure the gearbox has a full supply of oil before rowing through a bunch of 7k rpm upshifts :)
Update:
I found this page with specifications for all Audi/VW oils. The differential oil is specified as a 75w90 but the transmission oil does not list the weight. The longitudinal transmission oil does say that it's a 75w90 GL-5 synthetic so the transverse oil is probably similar.
