There's a different answer each time you ask, because there are different answers depending on application. Just like motor oil, you need to match the product with the application.
The big advantage to synthetic is that it can withstand heat better. Heat is generated by heavy loads (like towing a trailer or frequent hard acceleration) and high speeds.
Most people do not drive their cobra's under conditions that would generate high heat, even for short periods of time. But if you're racing on a road course or on the open road, you will heat that diff oil up pretty fast.
Also, the "weight" of oil depends on usage. The heavier weights protect the gears better, but tend to create more drag and parasitic losses. Like most things in life, it's a trade off. I use a heavier weight of oil for the race track. The parasitic losses are probably costing me some HP. But it's also protecting the gears a little bit better. I hope. Maybe. In theory. I have 6 years of racing and cruising on this differential. When I looked at them last fall, they looked pretty good, hardly any visible wear on them at all.
