Changed the diff oil yesterday and the front was a nice golden brown (it was overfilled from the PO), but the back was black as black can be. Everything was working fine before the change, but wanted synthetic in there before the winter comes. Used Castrol synthetic 75w90 with lube additive.

I'm assuming the blackness is from the clutches, the oil flowed nicely, even for sitting overnight. Do I have anything to be worried about?

P.S. Those are MONSTER gears in there

 

Yes, the LS axles will make the fluid darker.

 

actually its the friction modifier that makes the oil darker

 

A good synthetic fluid to use is amsoil 75w140 if you tow, or the 75w90 if you just drive on the highway. The amsoil already has the LS additive in it. I just changed my diffs and used the 75w140 in the rear and the 75w90 in the front.

 

Is it?
I never heard that.

 

You should see an 80...

 

How many miles did you have on her since the last oil change? I'm asking becasue I have 80,000 on my 2001 4x4 and I am changing out the front and rear fliuds with Mobil 1 syn (75w-90, don't do a lot of towing) for the 1st time this week. I suspect black might be too light to describe what I'm going to find.

 

I have 226,700 km on it, bought it last fall and don't know when the PO did the change. I put at least 16,000 km on it and the front was still nice and golden colour.

It seems to roll easier with the new oil in it. The reason I went with 75w90 was the -45C temperatures we get and the glue that was in there dragged allot.

 

My Dana 80 with the Mobil 75W/90 was only a golden brown at 230,000 kms. That is when the kid chipped the tooth on the leeding edge of the pinion and it has been out of ballance since.

 

How did they chip the leading tooth?