I've used Slick 50, Prolong and other snake oil products like these. I ceased using these products when it was shown that they were basically useless . . . and actually could cause problems.
One of the problems to my car was an intermittant 'tick-tick' from one lifter on the right side of the engine. Unfortunately it wasn't due to a broken oil tube (Menu#19).
Started using Auto-RX (ARX) and have been using it since. It cleaned all of the 100K+ miles worth of sludge and most of the varnish from the engine although it's not marketed to clean varnish. ARX does remove sludge amazingly and I continue to use it now in the maintenance mode. My engine has never run better and is quiet.
Here's a link showing what ARX did to a
'93 Nissan 240SX. The results were amazing especially the increase in compression for all cylinders.