The rear diff on the WRX\STI specs a 75W90 weight gear oil with no friction modifier, my question is why is the friction modifier not required? Any ideas?

And what would happen if you did introduce a friction modifier to the rear diff? Improvement, harm, nothing?

If you have an STi use the Subaru lsd 90wt. fluid.
$18.XX for a 1 liter can at subarugenuineparts.com
Your diff will thank you.

It's cause they need a certain amount of friction for the differential to operate correctly. If you reduce the friction too much...then it won't work.