According to the shop manual, you do use 75w90 synthetic. The factory already put synthetic in though. There is no recommended change interval, except for peace of mind.
Here is how much you need...
01E (6 spd Manual AWD) = 2.3 liters
01H (6 spd AWD Rr final drive) = 1.9 liters
Total to do both = 4.2 liters
01A (5 spd Manual AWD) = 2.75 liters
01R (5 spd AWD Rr final drive) = 1.9 liters
Total to do both = 4.65 liters
012/01W (FWD) = 2.25 liters
Remember that trans and rr drive are seperate.
I took this info from the AESIS(Audi USA repair manual)
If you want amounts for automatice, email/pm me.