A stop leak oil additives is a chemical formula which contains a mixture of friction modifiers along with some other refined petroleum products.
This non-solvent formula has a special purpose for the cooling system. Often due to mileage, heat, pressure and age, engine gaskets, seals and O-rings can shrink or become hard.
Now, this may lead to holes and openings in the engine from which oil can leak. The stop leak additives are such a formula that is designed to reduce the amount of oil getting leaked from worn out seals.