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!!

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"If God had meant for us to walk, why did he give us feet that fit car pedals?" Sir Sterling Moss

Michael
2014 E63S Estate
2006 SLK55
1995 E500
1986 Porsche 944 turbo