As a direct result of slick 50 I had to rebuild the engine in a Chevy Z71 4x4 that I had.
The engine was fine, and then ~ 3 months after putting Slick 50 in, I started noticing a heavy puff of smoke at startup and after sitting at a stop light or such. Over the course of about 2 weeks that went from non-existant to really bad. After the 2 or so weeks I started noticing a pinging in the engine too.
Well, I figured it was the old thing that those engines were notorious for... bad valve stem seals. So the next saturday, I tore into the engine to do that job, but as soon as I got the valve covers off, oil ran out in the floor... even after sitting all night. The engine was plugged up so bad that oil couldn't even return from the top of the engine.
I had that truck off the road for nearly 6 weeks. It took me every evening and all weekend for the entire 6 weeks to drop the oil pan, pull the cylinders (2 of which were cracked from wrist pin around to the other side), replace all the bad stuff and CLEAN.
The best of the cleaning I had to do was when it was black and flaky, almost like ash. That was probably only 10%. The rest of everything else was the consistency of tar and stuck to everything.
That broke me from using oil additives, ever again.