Fresh fluid will have MORE additives than the old fluid, therefore fresh fluid is better for the seals.
I know some people think that fresh fluid will ruin an old tranny; I disagree, fresh fluid will never hurt a tranny that is in good condition. If it fails after a fluid change, it would have failed w/o the fluid change. Its just convenient to blame the fluid when the real casue was the previous 90,000, ( or 120,000, or whatever) miles.

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1982 300SD " Wotan" ..On the road as of Jan 8, 2007 with Historic Tags