I used the redline 75w90ns and it seems to improve feel and be working fine. having said that, I will most likely go to oem for my next change. I think the Porsche change interval is too long (similar to their oil change interval) and like to use the 48k km interval recommended by pelican:
Pelican Technical Article: Boxster Differential / Manual Boxster Transmission Fluid Change - 986 / 987