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clean the spot and dye check it, you can buy dye check kits from welding supplys. if you have porosity in the castings super glue or epory will fix it. welding would be another option but i don't think welding would be my first pick. way back when i worked on cars for a living i fixed several casting flaws with something very simialr if not the same as super glue the the manufacturer recommended and supplied

 

If it's below the drain plug, it might just be that the plug is leaking... if so, take it out with a 6-point impact socket, and replace it with a brass one... use about 5 or 6 wraps of Teflon tape on it, do not use silicone.

 

it is past the plug, toward the tranny. about 1 inch past the plug