questions about changing diff. fluid?

[Clint]

Tommorow I am changing the front and rear diff. fluid on my 95 x-cab Z71 350 auto. My question is do I use the friction modifier additive or not?  I have the limited slip rear but the owners manual just says to use 80W-90 gear oil front and rear (nothing about a friction modifier).  In my Haynes manual nothing is mentioned about the supplement.  

 

I am waiting till the morning to buy the fluid so if anyone has a suggestion please post it.  I will be using either Valvoline or Castrol Hypoid 80w-90 gear oil BTW.

[chevy4x4trucks]

I know in the 99-up trucks no friction modifier is required beause of the way the locker is designed.  Not sure if it's similar on the pre-99 trucks.