Flushing of ATF is not recommended especially to older gearbox for 2 reasons, the flushing machine generate high pressure to flush the GB and may break some seals if the GB is old. Another problem with old GB is when u flush, u may have remove debris/sediment from the oil pan and it may get stuck in the GB when performance can be affected.
Periodical change of ATF and strainer, in this case, the oil pan would help to prolong the lifespan of the GB. Many recommendation out there, some say replace at 60k km (oil and pan), some say 40k km (oil and pan), I prefer 40k km (oil), 80k km (oil and pan)... hehe.. cheapskate..
The ZF Lifeguard 6 oil color is like our Cap Helang cooking oil

, brownish gold color. Do not get the 6 plus which is blue color liquid, that's not for our ZF 6HP19 box. The ZF 6 is retailing outside at no more than RM60/L and the plastic oil pan c/w strainer is no more than RM500 and it looks like this:
The E90 pre-LCI is using the same gearbox as E60 N52 engine. ZF 6HP19. Oh... meanwhile, I heard the plastic oil pan has a metal variant.. and cost no less than RM1.2k.. but I wonder how the strainer is attached and how heavy is the pan compare to plastic...