Yes, it IS a useful leak fixer, used in Subarus for leaky head gaskets. It
works well for dripping WBX head gaskets as well, many folks in the Vanagon
world use it. I've used it in mine since I bought it in 2016, the head gaskets
were weeping and quickly stopped after adding 2 bottles and haven't leaked
since. I add a bottle every other year or so. It's actually a product called
'Holt's Radweld':
https://www.holtsauto.com/products/radiator-repair/radweld/