I decided to change the transmission fluid in the Aisin transmission connected to the mid-engine, after seeing how many of the Mini Cooper owners have done this. Since some Mini platforms share a similar B38 engine and transmission design to the i8, their information was very helpful. Their fluid was coming out brown and so was mine, after 32k miles.
https://www.northamericanmotoring.co...n-6-speed.html
I could only drain 2.75L of the 5.5L capacity, due to there being much more fluid hiding in the torque converter, etc. I sent a sample to Blackstone for analysis, like the Mini owner did in the link below.
https://www.northamericanmotoring.co...000-miles.html
I'm considering doing this every 10k miles so newer fluid replaces older fluid, over time. The BMW ATF6 fluid that is recommended is cost prohibitive for doing this work. I did lots of research and the Febi Bilstein 29934 is a fraction of the cost and meets the same requirements for ATF6. Outside the US, some countries have access to the Aisin Premium ATF 6, which I couldn't get here in the US, or Aisin T-IV, but Febi was available at FCP Euro, so I'll do another drain and fill in the future and can send back the old fluid to FCP Euro for a 100% refund.