I would not suggest putting anything 'stop leak' in an automatic tranny. It's unlikely to work, and actually fixing the leak will be far cheaper than replacing the tranny.

See if you can figure out where it is leaking while you are under there. Check the lines to the cooler and the seal around the output shaft for starters. Maybe you will get lucky, and find the leak is around the pan gasket.

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1986 300E 5-Speed 240k mi.