The simplest are ones like Water Wetter are basically surfactants so they help a little. I found it to need to be replenished every few months when using with straight water or it would start getting rusty. The alternative is to add some water pump lube (Prestone makes one).
Royal Purple Ice has a good rep from what I recall. It is supposed to be reformulated, I think. Good for cars with aluminum radiators, etc.
As I mentioned above, I use RMI-25 which I order by the quart and use 8 ozs at a time per car (suggested to use 16 oz with straight water for a yearly use). It works well to soften and remove deposits from the block as well as well as act as a surfactant and lube. It does a good job on radiators to prevent old antifreeze from precipitating out in the radiator tubes or block. The block and radiator will stay very clean if you use it. Developed for long haul truckers.
I usually order it online for the best price and it is hard to find unless you go to a heavy truck shop.
But, yeah, it all works. I would say five to ten degrees is common although some claim more. Works best in a clean block and that is why I like the RMI-25. It gradually cleans deposits out. You will see the scum floating on the surface when you remove the cap.
