https://www.redlineautoparts.com/bmw...tic-83k-z4011/
Link above is same diff I have, there is a drain and fill hole.
Interesting thing about the OEM naca duct diff cover is that it directs air to the cooling fins, but right behind the finds is the chassis crossmember. The air essentially hits a wall. I did create a downward-scoop to help move the air back down to the ground (this didn't seem to make a difference since I am starting to see melting occur again on the new poly bushing)
I may try to redesign some ducting to help get the air OUT of that area more efficiently. Z4M's ducting seems to be ideal but it breaks up the smooth floor I designed.
I attached some examples of Z4 3.0si VS Z4M rear cooling ductwork. It seems the 3.0si naca duct is substantial for the fins, but there is not exit ducting even from factory.