Which diff are you referring to, the Torsen diff, which takes a specific super expensive oil as yes it can be changed, or the open front diff, which again yes can be changed. Here are the volumes and the different oil chambers
9l trans
1.1l front final drive
0.5l transfer/Torsen
1.5l rear final drive
I had a quote from audi once for the oils and quickly decided against it, I will see if I can dig it out somewhere