Just did the transmission fluid change on a 2009 135i with the 6 speed manual transmission, and took some pictures/video along the way. It's actually a very easy process to do. I used Redline D6-ATF fluid.
After driving it, I noticed that the shifter slides into gear more easily, and the notchy feel has been reduced particularly when it's cold.
I took the advice of 'wolfe' on this forum and took a sample of the old fluid. It certainly looked dirty after about 20,000 miles. Both the old and new fluids seem very thin compared to typical gear oil (like 75W-90) just from how fast they drain.
Hope someone finds this useful - comments welcome!
2009 135 Manual Transmission Fluid Drain and Fill