Limited slip diff., how to tell

[rickjordan]

This is another question for a friends car. I am going through the car, checking the brakes and changing the fluids.

Per the FSM and owner's manual the rear diff takes 90W gear oil, unless it's a limited slip differental. In that case it requires a "special hypoid oil"

How does one tell which diff. the car has. The car is a 1974 450SL

I will add that when playing around with the brakes on the back end, I found that I was able to rotate one wheel, with the other on the ground. this of course leads me to suspect it has a limited slip diff. If I am correct, what is, and more important, where can I get this "special oil"

Thank you again.