Synthetic Diff lube
For your consideration, I use synthetic in my diffs (Red Line 75W90) and it's worked perfectly. Since synthetic has superior properties to any dino oils, I run it in both cars (500E with some limited slip, 300TE with open diff) and have had zero problems in 130k miles (100k wagon, 30k 500E)and counting!!
"If everything seems under control, you're simply not going fast enough" --Mario Andretti
1995 E500 street car
1986 Porsche 944 Turbo S track car