Latest update. Attached are some pictures.
They now claim that the leak is coming from the side cover. Whether or not the bottom oil pan should have been replaced back in April remains unknown.
I want them to admit that the first repair did not fix the problem. A new leak should not occur so soon after the first repair. I also want to see if I can buy extended warranty or insurance after the repair.
Here is the full story.
In late march BMW chengdu checked the car, no oil leak problems found. (maybe at 25.000km)
In late april BMW tianjin (another dealer) identified that the transmission leaked oil. They randomly found it because I went there to buy a new battery for the car. They said it was a minor issue and normal for 3 year old cars...
BMW tianjin repaired the transmission shortly after at my expense. They replaced the bottom oil pan. (maybe at 30.000km)
Later I took the car in for engine oil change and other minor regular services at different BMW tianjin shops during the summer. No problems mentioned...
Now I asked another BMW dealer to check specifically if the transmission leak was fixed properly. (Because of this forum). They found that it was not fixed, and that a different part should be replaced. (now 39.000km)
Little known facts about BMW: 1) The N54 is still in production and used in the current Z4 35i and 35is. 2) In a Z4 you can get sport+ steering combined with normal suspension etc by selecting DSC=off. (Update: DSC=off also offers sports steering in the M135i) 3) All Z4s with 19" wheels are still only offered with the worst performing OEM tires ever sold with a DCT equipped BMW (Potenza RE 050 RFT).