We've had this discussion many times perhaps because people speed-read the prior posts or don't bother to search for them.
The Porsche specs for the 6 speed trans oil are kindly supplied by Markus here:
http://www.smithandallan.com/documents/Spirax_S5_ATE_75W-90_%28en-GB%29_TDS.pdf
This Thread may also help:
http://986forum.com/forums/general-discussions/55977-2nd-gear-hard-shift-after-tranny-oil-change.html
Of course we are always interested to read about experiments with new options but probably better to know what prior discussions have already eliminated ?
IMHO the expensive gearbox oil is the least significant of the issues with the 6 speed. Try the 2nd gear dog ring and synchro replacement - that really needs a price reduction and a diy solution !