Hi all,
time to replace the diff oil in the old cat. Which grade diff oil should I use?
Thanks
85W-90 I believe but I used 85W-140 the last time I changed mine and it seems to work just as well.
Thanks bud.
Glad to help. And BTW, the 85W-140 I bought was a oversight on my part but then I thought it might make the rear a little quieter and went ahead and used it. It did not make a difference in the quietness of the rear. Not that it is loud but it makes some gear noise under certain circumstances.
The heavier weight gear oil is Ok, but is designed for heavy duty truck rear ends. That weight is what we run in our trucks. The heavier weight is for the big ring and pinions under more load carrying weight and creating heat. The disadvantage of running it in your car rear end is more drag from the heavier weight gear lube. This will rob power and reduce fuel economy. In our trucks running that weight air temperature makes a big difference as well. Between a 80 degree day in the summer to a 10 degree day in the winter. Our trucks on average will burn about ten gallons more diesel because of the extra drag from the colder gear lube.
Thanks for the info guys, having had a good look around the pumpkin I can’t see a drain plug?? 
I can see the filler plug. How on earth do you drain the oil?? Do you have to split the diff? I hope not!! 
Sorry to say, YES.
Nooooooooooooooooooooo!!! How annoying, hmmmm has anyone ever considered drilling and tapping the pumpkin to install a drain plug? I.E can it be done?
I have not actually done this but I have been told you can remove one of the studs at the bottom and drain it fairly well. Anybody know for sure?
Good call, I’ll try that tomorrow bud and report back with my result. 
Think I went after this very notion and (OH NO!!!), the studs are “nail-heads”, i.e. the back part is bigger (like a wheel stud) and it ain’t no coming out the front! Not sure if all are this way but my two housings were!