Huh.... are you sure about the M-diff
http://www.bmwmregistry.com/model_faq.php?id=21
"What is the M Variable Differential Lock and how does it work?
The Variable M Differential Lock, fitted as standard equipment to every E60 M5 sedan and E61 M5 Touring, is an advanced version of the traditional limited slip differential that was jointly developed by BMW M and GKN Viscodrive. Though still entirely mechanical in operation, the M Variable Differential Lock is activated when there is a difference in rotational speed between the two rear wheels. This difference pressurizes a viscous silicon fluid, which then operates a multi-disc clutch that directs more power to the wheel with the greatest traction."
Think Killerfish is correct!