microSD will work, but is not recommended as its a two part object, vibrations in the car might cause it to seperate, then you've got 1 part lost somewhere.
Best to stick to a full size SD if possible. The owners manual does recommend a 1 piece
SD card, and warns against using a microSD with adaptor, due to parts getting "stuck".
More:
http://www.audi-sport.net/xf/threads/sd-card-for-music.234401/#post-2346750