Changing rear end fluid posi or not

[truckin 79]

Did a 79 1/2 ever come with a posi rear end? I need to change gasket and fluid but did not know if I needed to add special posi fluid. I have a big oil sludge hanging under rear end and so I think it is leaking gear oil and needs changed.
thanks

[Matt Man]

Could have came with a g80 which required the additive. Once you have the cover off you will know if it has one or not.

[mongocanfly]

Does it still have the spid in the glove box?...

[cadillac_al]

I always used Carquest gear oil that works in either one.

[CSHADES]

We started using the Ford posi additive to anything with posi traction.

[truckin 79]

I do not have spid had to change glove box doors as hinge was gone. I just assumed most later trucks with be a one legger and no posi. I heard that if you go with Synthetic a person does not need to posi additive. Is this true?

[Matt Man]

Originally Posted by truckin 79

I do not have spid had to change glove box doors as hinge was gone. I just assumed most later trucks with be a one legger and no posi. I heard that if you go with Synthetic a person does not need to posi additive. Is this true?

No synthetic fluid does not replace the posi additive.

This is a open rear end:


This is a g80 rear end:


The g80 in the half ton axles (12 bolt, 10 bolt) are not very good I would be surprised that it would still be in there. My 81 came with one but when I got the truck it was not there, just had a open diff.