Suregrip additive?

Pennzoil 4096, Gearplus 80W-90, now sold as Pennzoil Gear and Axle. Used and sold it since 1985. Limited slip additive already included in the formula. Never had a customer complaint about chattering, although probably more than half would have been used in open diffs. Wholesale price was pretty cheap, but retail markups can be very high. Yeah, $6.25/qt by the case of 12 on Amazon. $15.52 for a single quart.