I have an 03 a4 quattro 1.8t, I have a sponsership thru motul and need to know if I can use synthetics in the diffs , also what gear oil weights?
also to service the trans how much fluid is put back in and thru where

 

Yes you can use synthetics, in fact they are recommended as the dealer uses a castrol synthetic gear oil. 75w-90 i believe is the correct weight in the diffs.
The trans usually takes around 5-6 liters, but you need the correct fill tool and a vag com to monitor the transmission temp to know if its full. Basically you fill it from the bottom and it has to be a certain temp, when it reaches that certain temp the fluid will start leaking from the bottom, thats how you know when its full. If you don't have these tools have the dealer do it.