Q: Will adding Transmission Fluid with Stop Leak to a transmission with over 230,000 miles hurt it?

Both of these additives are designed to block small leaks or seepage from a gasket or rubber seal an not to fix major leaks or leaks where there will be excessive pressures, such as coolant leaks into the combustion chamber of the cooling system or leaks in high pressure areas of the transmission.

I would not recommend the use of these products unless a manufacturer specified to use them, like how GM did on their earlier models of the modular motors that had coolant leaking through the pores of the engine block from defective castings.

Before adding any of these additives I would recommend having a mechanic check the transmission fluid leak to see what the proper repair should be. These additives can sometimes clog systems and make matters worse in the long run.