Thanks Robert, was waiting for your reply.
I really don't have much in the way of comment regarding Zmax in particular, but in general I don't see an actual need for the product, or similar products, in the first place.
I guess the short answer is I don't understand what problem these products are trying to address. I don't have any issues with engine reliability or longevity with regular oil and filter changes. a little attention to cooling, not being afraid to pop the hood every once in a while and having a poke around, and a habit of always starting the engine with everything (A/C, heater, radio, etc) "off" and listening to the motor for the first few minutes before adding the noisemakers.
I see guys seemingly worried about relatively new engines; I don't have much to say there since aside from my motorcycle I've never owned one in that condition.
But I will say that I always buy used vehicles, and I've never lost an engine (total price paid for 5 used trucks + 1 car: $6350; HD about $8500 invested; Miata $2000 and PT Cruiser $3200). I use 0W-xx semi-synthetic or synthetic **, change oil and filter once a year, and leave it at that as far as worrying about the oil goes. I put about 6~10,000 miles annually on my daily driver; the lower figure is due to my work which results in my vehicles sitting for 3 months unused at least once a year. That is not necessarily good for an engine, so I wouldn't call it light duty.
I use the 0W-multigrade because my vehicles experience cold starts at air temps up to and below -40 every year. (Note: Wind Chill has nothing to do with engine starts; air temperature only figures cited). The coldest start was @ -46C (-51F) with the GMC from a storage lot in the Arctic, no block heater help available. That is severe service in anyone's book.
I normally don't have access to electricity where I park, so block heaters are rarely used. I expect the vehicle to start every time, regardless of the weather. I don't use dual batteries or any special starting aids, but trust me your vehicle needs to be in good working condition or you're not going to be able to start it.
Summer temps range up to +40 air (105F). I run the same oil year-round.
I actually own three licensed vehicles right now, normally I previously just ran a truck (1/2 or 3/4 ton). I've switched to running a car and truck as a daily driver for fuel consumption reasons two years ago.
My current truck was bought with 230,000 km (143,000 miles) just recently. It replaced a 77 GMC 350/T400 that I bought with 186,000 miles and ran for 8 years. It has transmission issues but the engine will fire right up this second, currently with 265,000 miles on the clock. It uses no oil to speak of, perhaps l quart per year. I've had slightly better gas mileage from similar 70's GMCs but this one was adequate at 20 hwy and 12 city (per US gallon), and was consistent over the time I owned the truck.
I have a PT Cruiser I bought with 175,000 km (109,000 mi) and a 20 year old Miata that has 275,000 km currently (171,000 mi). The Miata regularly sees shifts at 72~7500 RPM and occasional sustained high RPM operation (up to 3 hours).
I expect all these vehicles to last many more years; I've always bought used, and always wore out the vehicle itself rather than the engine. I have never lost an engine in a vehicle in my life, and I get good life from them: six used vehicles over a 27 year span (one was burnt by a pyro who was torching cars and trucks around town so only ran it for 2 years). I've never failed to pass a vehicle inspection save for minor issues (broken parking brake line; licence plate lamp out, that kind of thing; you get a pass if you show the necessary repair was done within 30 days).
The motor is in my opinion the most reliable part of a car or truck.
So, I don't see exactly what people are trying to achieve with these additives. It seems like a solution in search of a problem to me.
The local Dodge Store just ran an ad with a guy who's on his third RAM Diesel 1-ton. Just traded his old one ... something like a 2005 model ... with over 1 million km on the clock, for a showroom-stock '12 model. His previous truck had 900,000 on it; he sold it to a buddy rather than trade it in. Regular factory fill oil and filter at the suggested intervals.
Naturally this is all anecdotal evidence, so grab your box of salt, but I do have to say that anecdotal evidence is a primary marketing tool of the additive companies, so I say "apples vs apples".
** Esso ArticLube 0W-30 Semi-Synthetic 1985~1991
PetroCanada 0W-30 Semi-Synthetic 1991~1996
Mobil 1 0W-40 Synthetic 1996-present
If I didn't need the 0W oil for winter starts, I'd probably use different oil.
The Miata gets 10W-30 because it doesn't see winter use.
The bike started off with straight 50-weight (factory recommended viscosity) AeroShell W100 (AD) because I got a 20-litre pail for free, now it uses 20W-50 motorcycle conventional/synthetic depending on what is on the shelf at the time for a reasonable price.
After a rebuild, intervals were 50 miles, 500 miles, then 2000 miles (AeroShell; aviation AD oil is non-detergent) now 3000 miles (actual motorcycle oil) with a change in the spring or fall depending on mileage since last change.