You got a nice 10%+ increase in mpg, but did you notice how long the effect lasted? I too got some mpg increase (7-8%), but the effect seemed to wear off gradually, but rather quickly, as if the zMAX was evaporating out of the crankcase, or out of the metal of the engine if you believe zMAX. Whatever, it seemed to me that the cost of the zMAX was hard to justify because of the "evaporation slope". I think I remember that the majority of mpg increase effect was mostly gone after 3000 miles after adding. What was your experience with this issue?