Just got this from Valvoline's website:
Full Synthetic Gear Oil
Formulated for ultimate protection and performance
Valvoline SynPower Gear Oil is a full synthetic, thermally stable, extreme-pressure gear lubricant designed to operate and protect in both high and low extreme-temperature conditions. SynPower Gear Oil is specially recommended for limited-slip hypoid differentials.
Contains additives to assist in protecting gear teeth
Maintains excellent low temperature fluid protection down to -50°F (-46°C)
Provides outstanding thermal stability for cleanliness and longer service life
Contains special additives to reduce chattering in limited-slip differentials
Recommended for (conventional and limited slip) manual transmissions where an API GL-5 or MT-1 fluid is specified
Protects parts from rust and corrosion
Compatible with conventional gear lubricants
So according to this you don't need any additives....