Don't use a stop leak that is in an oil base. Barrs and others sell that. The oil can interfere with the antifoam additives in antifreeze. So be sure to use a dry powder.

General Motors, for over at least the last 70 years has put stop leak powder into their vehicles as a preventative measure. They use ground up ginger root. There used to be a Vernor's Ginger Ale factory in Detroit. Someone found that ground up ginger root would seal up small leaks but not harm the water pump or other internal engine parts. You can buy that stop leak in pellets about 1" in diameter and 1/2" thick. Usually in the plastic bubble packs at the parts store.

GM would put two pellets into a V-8 and one into a six cylinder on the assembly line.and
They are sold under the AC/Delco label and also other brands. Here is a picture on Amazon: https://www.amazon.com/s/ref=nb_sb_noss?url=search-alias%3Daps&field-keywords=stop+leak+pellets

Back in the 70's both DuPont with their Xerex antifreeze and Dow Chemical's antifreeze advertised and sold jugs with stop leak already in the antifreeze. DuPont used tiny styrene plastic pellets (which were made by Dow as water softener resin) and Dow used ginger root powder.

Emery Stora
77 Kingsley
Frederick, CO