I went under my car today to drain the rear diff and i found the drain plug but could not see the filler plug. is their one? I also want to change the gearbox and front diff oil but I wanted to see if this is even possible.
Thanks.

 

I did the rear diff the fill plug is on the passenger side just about an inch behind the the axle(it may be the driver side, I dont remember for sure but it is right behind the axle). This bolt for me was extremely difficult to get out I snaped one allen socket (10mm) and twisted another untill it finally came free. I actually used the jack and lifted up on the breaker bar so the car started to come up off the ground with the weels still barley touching and soaked it in kroyl, left it like this over night. This was after snaping the first socket. The next day was still a bitch to get free but finally came loose.
To fill I used a LARGE syringe and clear plastic tube about 3-4 feet long. It takes quite a while to do, leave the tube in the diff and remove the plunger on the syringe and fill it with the oil then plunger it back in. I believe it takes 1.9qt.

I also did the trany/front diff (manual not automatic). The drain plug is easy to spot its the only one for the trany under the car. The fill plug is on the driver side just about an inch forward the axle and uses a special tool to remove the plug. I didnt have as much trouble removing this plug. Ill try to find where I bought the tool from. Fill this one the same way as the rear diff using the syringe and tube. I think this takes about 2.9qt.

I assume you have an S4 from you name I did this on a manual trans A4 I think the plugs are in the same location provided you have a manual trany.

 

found the tool

http://www.zdmak.com/wbstore/main.asp

type in 3357 and do a search

I went with the cheaper one.

I think this was for the drain AND fill plug for the trany on mine. I dont remember for sure if it was one or both, but I did need it.