"Shockproof" is a line of gear oil Redline manufactures for racing. According to Redline, it contains a unique solid dispersion which cushions gear teeth to help prevent tooth breakage and allows the use of lower viscosities. It comes with three versions: Heavy, Lightweight, and Superlight.

When my 1991 300E developed some differential whine, I communicated with Redline and advised me against the Shockproof products (although it will quiet the differential the most) because of the street use nature of my car. The Shockproof Heavy is essentially a 80W-250 gear oil.

In stead, Redline recommended me to use the 80W140 Gear Oil. It is for heavily-loaded and low-speed differentials, including limited-slip, and where reduction of limited-slip chatter or gear noise is required.

I bought two quarts and they really quiet down the differential. The whine is still there but much reduced.

Before I bought the 80W-140, I tested with the Wal-Mart brand dino 85W-140 and that quiet down the while as well. But being so thick at hot (140), dino is not a good idea.

The other thing you can try is to get Vavoline's Synthetic blend 85W-140.