SWEPCO is a known "racers" brand, and it has proven itself on Porsches in the tranny and diff.
The Porsche 928 users use SWEPCO in tranny for better shifts (the automatic is a MB tranny), and in the diff it has saved those with even a missing top plug and only filled to half-capacity.
I personally use the SWEPCO 201 80-90WT for diff (includes limited-slip differential modifiers), and the SWEPCO 714 20WT for the automatic tranny.
I get from the RiteGear Porsche site:
$36.50/gallon for 201 diff oil
$19.95/gallon for 714 ATF
Best of luck,