I tried it in my car, I did not appear to notice a difference, but I put it in within the first few days, My head gasket had a slight slight leak, tiny, like pinhole sized, and it has not shown any more of a leak than before, infact, I think it stopped leaking all together. My mechanic looked at it and found no problems with the gasket leaking even though when he first looked at it when I bought the car he pointed out a small leak at the front that was basically just an inch of grime near the head gasket, which appeared to me to be slight oil, burned nicely by the block.
Either way, I did not see it cause any more of a leak, and I can not tell if it made an improvement, but my mechanic loves the way my car drives.
And I did the full treatment of Zmax, and My Tranny went out on me last week after having driven about 8k miles, but the zmax could not have been the reason or anything, because I lost my clutch packs on 3rd and 4th, which were disintegrated int he tranny pan.
But with the new tranny rebuild, the trans is so smooth and shifts so nicely, that I can tell my car needed it when I bought it. The Difference is like night and day.
So form my experience, I did not notice any difference with zmax.
I had also tried dura lube but on my jeep and it did not appear to do anything, but it did not appear to hurt anything either. I think these additives, while some may work decently, Are more about marketing than actual results, but I think if it gives you peace of mine to have it in your car, and if it works for you, then use it.
I don't think the Zmax will hurt your car, it hasn't appeared to have hurt my car or others.