I have a 1991 300SE and I changed the differential fluid to a synthetic grade that Walmart sold as gear oil. Drives OK in city but as soon as I drove it high speeds for 40 miles there was a howling so bad I thought the rear wheel bearing was going out. This happened every time I drove it on the interstate.

So I went to the dealer and got their last liter of special Hypoid gear oil and changed it. What do you know, no more mowling and its spec. So in some cases it does pay to use MB Fluids.