Slick 50 works perfectly on older engines with large oil galleries (like Pug 504s) but it can cause blockages on newer engines which have very small oil galleries. Slick 50 is just teflon, there is nothing magic about it. You just have to be smart about where its use is appropriate and where it is not.
On my 404 which had a very noisy gearbox whine (due to being run on low oil), I put slick 50 in it and after 20,000kms or so the gearbox became quiet. I imagine that the teflon must have filled up all the pitting in the bearing races and gear faces which was causing the noise.