I think your is engaged in wishful thinking or now that he's added it he's suddenly more aware of his driving habits and is being more careful on the throttle (most people do this after getting mileage improvers).
Zmax has been around since 1947, and I seriously doubt some miracle potion made way back then before the advent of modern additives is going to live up to its claims. If some 60 year old cocktail alone could add 4 mpg the auto manufacturing companies would put in the engines from the factory.
According to the Federal Trade Commission the product is merely tinted mineral oil. This may thin out oil and give a slight mileage increase (I doubt it) and .... it can cause damage to the bearings. Not worth spinning a bearing or other engine damage which can occur from running too thin an oil.
"According to the FTC, the CRC L38 test is a standard auto industry tool to measure the bearing corrosion protection properties of motor oils. In February/March 1997, an independent laboratory performed two CRC L38 tests of zMax for Speedway and Oil Chem. In those tests, motor oil treated with zMax produced more than twice as much bearing corrosion as motor oil alone. The complaint also states that the defendants fabricated one "report" from the two test reports, eliminating the bearing corrosion results and all other negative test results, and then used that report and the "official laboratory results" - similarly edited to remove detrimental data results - as sales tools in the infomercial and on the zMax Web site."
I think he'd have better success, and safer too, with a synthetic oil of the right viscosity.
Read this before adding any miracle potion to your engine:
Snake Oil! - Is That Additive Really A Negative? .: Articles
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