Some stuff is worth keeping on the shelf. For me what makes the list are small parts that could shut down work if needed (oil drain gasket), or stuff that is cheap and might make work easier if we had a handful sitting around to be used when...
Big ole NOPE for me to any additive that "fixes" leaks.
The way those work is to use chemicals to make rubber compounds expand. This reduces their density, which softens the rubber (lowers the durometer), leading to increased wear. This happens...