About Slick 50: In 1996 the Federal Trade Commission charged Quaker State (the manufacturer of Slick 50) with making false and unsubstantiated advertising claims. According to the FTC ads for Slick 50 touted tests that claimed improved engine performance that were false, and its claims of reduced engine wear were unsubstantiated. The result: Quaker State / Slick 50 / other Quaker State subsidiaries had to stop making claims that it did not have competent and reliable evidence to substantiate, and was fined $10 million. Since then I've noticed very little advertising for Slick 50 (I was surprised to find that it is still available).
BTW, Quaker State's Slick 50 hasn't been the only oil additive taken to court by the Federal Trade Commission. It also filed similar complaints against STP Oil Treatment, STP Engine Treatment, Motor Up Engine Treatment, DuraLube, and zMax Power System. Each also resulted in fines and diminished advertising, although Slick 50 was fined a lot more than the others.
Prolong Super Lubricants got punished by the FTC for an ad featuring Al Unser. Supposedly an engine had been treated with its product and then run hard for a long time (without problems). Prolong pulled its ads and avoided paying fines.
Back in those days I was racing karts a lot and building my own engines. Those were Briggs & Stratton 5 hp 4-cycle engines that had splash lubrication (essentially lubrication by oil cling and luck). I tried several oil additives, including Slick 50, but didn't notice any improvements. Not claiming my experiments were fully scientific - and they were definitely not the last word on the subject - but I suspect my experiments and results were similar to other low-budget racers.