rear differential fluid question

just hit my 50,000 mile mark (yay!) and the dealer says i need to change rear diff fluid (which realistically probably doesn't need to). I have the part number 83 22 2 282 583 and I am going to do this myself. How much rear diff fluid do i need to buy and how much do i have to actually use? e92 m3

Thanks!

http://www.m3post.com/forums/showthread.php?t=714350