I was reading the data sheet on valvoline 80w90 and it states it has the Limited Slip Friction Modifier in the product. See below.
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. http://www.valvoline.com/pdf/high_performance_gear.pdf
Does everyone still add the friction modifier or go by what valvoline states.
No. I'm just getting this other rear axle ready and I had a couple quarts of the valvoline setting here. Just wondering if I should add the additive with it.
Stopped by our GM dealer and asked for the GM friction additive and he returns with a MOPAR brand. He said they were out of GM but the MOPAR was the same. I said never mind. I will order it on line. Not putting no MOPAR liquid in my Firebird.