When i first got my MGB i knew it had a very small leak – more of a seep – in the corner of the radiator. Also the engine / gearbox was very oily underneath.
The radweld cured the water leak but it was difficult to tell whether the oil leaks were cured.
When i took the engine and gearbox out i replaced all the rubber shaft seals anyway. But the main oil leaks were the engine sump and other gaskets which were replaced.
Additives won’t cure gasket leaks.
But they say there’s not an MGB on the road that has absolutely no oil leaks!!