I used redline in the last couple I had apart. The additive is a friction modifier which makes the fluid a little slicker so the clutch packs can slip a little easier when the wheels are turning at different rates. Add just a little, take it for a spin and add more if the diff feels tight or the outer wheel looses traction and chirps through turns. Any COMPETENT clerk at a parts store will be able to help you find it. I don't like buying oils with it already added when just changing fluid in an old diff as there may be too much.
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