The Lucas Stop leak additive and other engine oil additives work by softening the gaskets when they get hard and that is what makes the oil or other leak stop.
Although the Lucas Stop leak additive does work to stop engine oil leaks it doesn't happen immediately.
The Lucas Stop leak additive and other stop leak products work overtime through the chemical having contact with the seals to soften them up.
So yes they do work but they don't work right away. You'll still have leaks for awhile.
It's a good idea to get the leak actually fixed with a new seal if you do have an engine oil leak or transmission leak to fully fix the problem.