I saw that from your profile. Most quality GL5s already contain the LSD additive in the proper ratio for factory LSD units.
Look for wording on the label similar to:
"Suitable for make-up and refill of conventional and limited slip differentials...."
Refill being the key word
This is from Valvoline.
Valvoline High Performance Gear Oils are superior sulfur-phosphorus, extreme pressure gear lubricants formulated with premium quality base stocks to meet the demands for excellent performance. It is designed to provide excellent load carrying capacity, extreme pressure properties, anti-foam performance, demulsibility, corrosion protection, thermal stability protection, and service fill limited slip capability. These products are recommended for conventional rear axles, limited slip rear axles, and transmissions requiring EP gear lubes under high speed, high load, high torque, and high horsepower conditions. Valvoline High Performance Gear Oils meet or exceed API Services GL-5 and GL-4*. The inclusion of Limited Slip Friction Modifier in this product makes it unnecessary to add additional friction modifier (Ford M2C118A, Chrysler MS-5630, or GM1052358) in most vehicles
In reality, there isn't a whole lot of difference in viscosity between 75W or 80W and the exact viscosity we use in our axles is not critical. In theory however, the 75W rating would be a little better in the coldest conditions than 80W would but not by a significant margin. A 75W-90 or 80W-90 would be fine for extremely cold conditions.