Leaking front diff

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Hi, I recently changed oil in diffs, gearbox and transfer box of my 96 Indy.
I've now noticed an oil leak from either side of my front diff.
My first thought was to change these seals but can't understand why they should both go now with new oil? I did notice that the oil that came out was slimey looking. I replaced with ep90

New oil has a detergent in it

New oil has a detergent in it and this can clean gunk around seals and make them leak, for peace of mind change the seals.