It doesn't make any difference. The friction modifier is there to prevent clutch chatter. With of without, a proper GL-5 gear oil has the necessary EP additives that are needed for the hypoid gear set.

My recommendation is to use a mineral oil based 80W-90 GL-5 that you should be able to procure at any auto parts store. Eventhough the label may say "suitable for limited slip differentials" most DO NOT have friction modifier. One exception is Redline 75W-90, which does, but as you said 75W-90 NS does not.

Duke