When i first sucked out the trans lube on my SPF's 8.8 diff, to replace it with fresh lube, which you have to do by loosening the rear diff cover, not an quick task, as there is no drain plug on these, only a filler plug, it came out gray and thick. So i think the stock fill is some sort of gray
oil. I did it warm with my Mityvac. Later, when the Olthoff's did some drag race mod to my rear diff mounting brackets, and had my diff out, Dennis drilled a hole in the cover, and welded in a nice AN 4 drain plug, so now i can just let it run out, easy as pie. I am pretty religious about changing my drivetrain fluids on a timely basis, especially with all the track work my car is subjected to.
I have gone to Redline for my diff, on the third fill i think. But when i most recently drained it hot, i carefully inspected my fluid for wear particles. It had only a tiny bit of sliver metal, about a match head's worth. Plus, even with my forth set of rear tires on the car, as of this afternoon, my diff locking action is perfect. I can lay down two even strips of rubber, my rear tires wear evenly side to side, so my clutch disc's must still be hanging in there. No fish-tailing, etc.
So i think the 8.8 is a fine diff, and if yours has gone belly up, it happens. Please let us know how soon you are back on the road.
I drove up to visit with the Olthoff's this afternoon, a chilly but sunny 155 mile trip. They put on my forth rear set of GT II's, and wished me well. Always nice to see the shiny new cars being made road-worthy. I like to brag and show off my latest tweak and modification, now a lighted Accusump switch, so i can see at a glance if it is on or off. I am making arrangements to have a full width race roll bar put in, as i am too tall with my cone-head in a Simpson helmet for amateur track running at some of the courses i want to try out. The new full bar is about three inches taller, and is quad braced.
Mostly i am putting this in for my son's safety for when he hammers it. The trick is to squirell away enough money without the Wife finding my "secret envelope!"