rear axle lube color
 

When I checked the rear axle on my wifes 2002 Mountaineer the fluid was grayist. Is this normal or isbecause it'ssynthetic 75w-140 fluid? The reason that I checked the rear was because itseems to begetting louder.
Also, how can I tell if this car has a standard or a limited slip rear? I see that it will take a friction modifier if it has a limited slip rear. How much fluid dose this axle hold?
Thanks, Jay