Topic: Differential gear oil change

[bill6567]

Is it necessary or is it recommended to change the gear oil in the differential of a Vintage Mustang?

[66GTKFB]

If you feel ambitious, go ahead. If your rear axle has never been rebuilt, the oil is in the 40 year old plus range so consider that. Otherwise, top it off and concentrate on something else.
Jim 

[Soaring]

I disagree with Jim on this one.  Over a period of years, moisture can build up in the gear case, and with friction on the gears, the lubrication tends to break down and turn black, etc.  I changed my rear end lube to a 75-90W synthetic lube, and I also changed my standard transmission with the same stuff.  To me, it is just good insurance that the lube in the rear and tranny are fresh and the latest product that will protect them.  It was like $65.00 to do that.  Cheap insurance. 

[mustangluver]

how much gear fluid would i need for that. im lookin at goin synthetic.

[66GTKFB]

Before you get into this, you cannot mix gear oil type. To do this, you will need to flush out all the old oil. That being said, 5 pints is the dry capacity of either a 7 1/4 or 8 inch type axles, a 9 inch takes a bit more. The question is - why?
Jim
Correct the capacities: 7 1/4 inch (6 cylinder rear axles) - 2 1/2 pints; 8 inch - 4 1/2 pints; 9 inch - 5 pints.

[Soaring]

Jim is right about not mixing them and the capacities.  Take it to a lube place and have them suck out the old stuff and add the new synthetic.  You will immediately notice a difference because the synthetic is slicker and lubes much better than the old dino 90 weight gear oil.  I had that done to my 65 8" and my 3 speed tranny and couldn't be happier with the results.