If you have a manual Redline D6 will smooth out the shifts.
Go to the redlineoil.com site and they will tell you what you need
they have and app.
The only fluid that does not have an aftermarket substitute is the transfer
box fluid for awd I believe.
If you have and auto its way expensive to change the fluid.
Consult the Mike Miller old school maintenance schedule for guidelines
on fluid repair and maintenance . Google Mike Miller BMW for a list of links
on how to get it.
Also there are DIY's in the DIY and maintenance subforums
http://www.e90post.com/forums/forumdisplay.php?f=16