IIRC, Carroll Shelby even did endorsements for Zmax.
So far as I'm aware, none of those products have been shown to do anything besides take $40 out of your wallet. And some have even been proven to be harmful to your engine. Just run good oil and filter and change regularly, that's all you need.
2001 Laser Red GT convertible; all of the usual bolt-ons.
1967 Notch; 200 I-6, C4 auto.