1999 A6Q transmission leak

[Vic]

I have a 1999 A6Q with 148k miles on it that has started to leak transmission fluid. It is definitely ATF, not gear oil, and appears to be coming from the area of the right front drive shaft flange. At least, I hope that's where it's coming from since replacing the driveshaft seal looks like a very easy job. Dumb question - would a leak past this seal be ATF or would it be gear oil? I can't figure out which by looking at my Bentley. Obviously if the answer is gear oil then the driveshaft seal is not the cause of my problem.  Also, am I likely to lose much fluid while replacing the seal?

Thanks.

Vic Norman