Did you read the post above mine?
Ok I see, reading post without seeing facial expression, misleads the reader .. Was to close to the display got gear contaminate in my eye
I did a gear swap in my GT350 at around 15,000 miles. I saved the factory fill OEM fluid in a clean gallon plastic jug. I believe this fluid stayed on the shelf about 6 months. (With plenty of time for contaminates to settle). Upon inspection I noticed some black residue had settled to the bottom of the jug.
Here in the second photo I turned the same jug on its side and you can see the residue stayed on the bottom.
The ford drain plugs have a magnet built into them and this black residue has seemed to accumulate on the magnet of the drain plug on every diff I have drained, (And I have drained a few). I call it the nasty black oxide that wears off of the ring gear and pinion. The black oxide finish on the gears I believe is from the heat-treat process performed on the gears during manufacturing.
I have been advising mustang owners to change the OEM diff fluid maybe at 5-10k miles, and I would definitely do it when the fluid is hot, to get all the residue out. If you have a traction-lok make sure you have the friction additive added back in, or make sure your fluid already has it.
For some additional info: I have removed my diff housing several times due to testing new gear ratios and newly designed rear covers so I have been able to inspect the fluid coming out every time. Also on the diffs that I have worked on such as bearing replacements, or new parts put in with polished ring & pinions, the fluid that I have drained out later is as clean as you could ever want.
Its only purpose is to reduce possible chatter during slow speed turning. I haven’t used it in any of my 8.8/Super 8.8 swaps/fluid changes in 25 years. Never had chattering/noise and never had clutches wear/fail.
In general friction modifiers can change how slick or slippery the oil is, changing how parts engage one another.
If I'm not mistaken, chatter during slow speed turning could arise if one wheel wanted to rotate faster than the differential allowed it to. In other words, if the diff weren't open enough. This means the friction modifier decreases friction between the plates. So if we don't add the friction modifier, then the diff will be a bit more slip-limited than usual, as if we increased the preload.
