I can't speak specifically to Z-Max, but over the years I have seen many oil treatment products come and go. Whatever the product color, or color of the container, they all helped the transfer of green from your pocket to the sellers. My father was a sucker for all kinds of engine additives and could never identify any benefit beyond his hope. From my experience there is no substitute for regular maintence to achieve optimal performance.
Over the years I bought a lot of products like STP injector cleaner, Gumout, STP fuel additive, Slick50, Marvel, Chevron and so on... got the warm and fuzzy feeling that my engine is happy.... still sometimes buy some of this snake oil when it's on sale.... I never bought Zmax - $4 is my personal limit for BS like this.....