There are actually two (2) manual transmission fluids that are suitable and approved for Porsche transmissions:
First is Mobilube PTX:
MOBILUBE PTX 75W-90
Good luck however trying to find it.
The second one is Shell Transaxle 75W-90 renamed not too long ago to Shell SPirax S5 ATE 75W-90.
Data sheet here:
http://www.shell-distributor.ru/common/data/u/Spirax_S5_ATE_75W-90_(TDS-rus).pdf
I only found its Russian version, as a new Data Sheet with removed Porsche reference is displacing old ones on Shell web sites worldwide. However you can still see "Porsche" in the Russian version.
Hurry up Shell, remove that one too!!!
I actually bought 20L pail not long ago. The label clearly states that the fluid is Ferrari, Maserati and Porsche approvded which is consistent with earlier Shell published data sheets.
This fluid is available from Shell distributors in Canada (not easily) in 20L pails only. Not sure about situation in US.
Both fluids are under 600 cSt at 0*C (Porsche requirement) to maintain good synchro engagement in low ambient temperatures.
The early version was fully synthetic (Transaxle) the new one is Group 3+ XHVI Shell GTL basestock based fluid. It performs very well indeed in my 997 G-box.
Any other fluid will cause issues with 996/7 transmissions. More voscous fluids will be a hell to shift in cold (Mobil Delvac comes to my mind). Any less viscous, Motul Gear 300 as example is not recommended by Motul in 996/7 boxes.
Here is what I collected to date on Rennlist:
997 Gear Lubricant Data - Rennlist Discussion Forums
Cheers,
Luke