Jackd used a pretty broad brush in his swipe at the bottled miracles and supershine number 9 and so on.
I think Amsoil, K&N and RedLine offer a variety of excellent products. I judge most of the products I buy based on data I can get without generating it myself, and in some cases I admit I can be swayed to try one item based on having had good experience with another of that manufacturer's offerings.
I use RedLine's array of gear lubes, Diesel fuel catalyst, and Water Wetter. While I am not in a position to conduct an engineering study using my car to figure out that Water Wetter does or does not work, I believe it does reduce the tendency of the coolant to cavitate based on first hand experience. I used it for the life of my 1986 190E 2.3-16, which lasted 205,000 miles before it burned an exhaust valve and had to have the head off for a rebuild. In its earliest incarnations it was a substance like pink Bromoseltzer and you had to add this solid stuff to the radiator.
Anyway, when the aluminum head came off there was nothing in the way of chemical deposits or pitting on any visible surfaces in the coolant passages. The tech working on the car noted the head was preserved better than expected, as he was astounded the water pump and the head looked nearly new after 200,000 miles. I replaced the water pump just because it was much easier to get to with the engine torn down, and I could not believe it was going to last another 200,000.
I am also reasonably certain the data on RedLine's site is reasonable. The theory behind the concept is good physics, and the fact that the lack of cavitation damage noted as unusual by the tech (very good qualifications, came recommended from another member of this forum) supports the wetting function works, so I believe the improved heat transfer works too.
I seriously doubt most installations will result in lower temperatures on the water temp gage that is witnessed by the operator. This is because most of the time the conditions the car is operated in do not run up to the maximum heat removal capacity of the system, and that is the only time when a lower temperature is likely to be apparent.
For example, when the coolant temperature of the system is controlled by the thermostat, the presence of WaterWetter will not affect the coolant temperature. The same is true when the limiting factor is the thermostat malfunctioning, or the viscous fan, or the electric fan clutches not working (not enough air to remove the heat, even if it gets to the radiator faster or more efficiently, will not lower the temps), or some other system element failure.
So, do not expect WaterWetter to make up for a failure of some part of the coolant system, and do not expect to see a real temperature difference on the temperature gage until you get to the point with the system where everything is working at full capacity, and correctly. In that case you should see a temperature difference if WaterWetter in fact allows the coolant to pick up heat in the head and transport it to the walls of the radiator more effectively. This characteristic is shown to exist in the data on RedLine's site, but you have to understand what it really means. In the very least, it does reduce the propensity for cavitation, and it will therefore limit the existence of hot spots due to unfavorable flow characteristics that come along with cavitation and the conditions that lead to cavitation.
The point is though, if some part of the system is maxed out by either capacity or some degradation, WaterWetter will not cure the problem or increase the system capacity. I hope this is reasonably clear. And I have no affiliation with RedLine whatsoever, I just like their products, including WaterWetter. Jim
1986 Euro 190E 2.3-16 (291,000 miles),
1998 E300D TurboDiesel, 231,000 miles -purchased with 45,000,
1988 300E 5-speed 252,000 miles,
1983 240D 4-speed, purchased w/136,000, now with 222,000 miles.
2009 ML320CDI Bluetec, 89,000 miles
1971 220D (250,000 miles plus, sold to father-in-law),
1975 240D (245,000 miles - died of body rot),
1991 350SD (176,560 miles, weakest Benz I have owned),
1999 C230 Sport (45,400 miles),
1982 240D (321,000 miles, put to sleep)