There was a thread on this topic last year - there are several of us using Redline Water Wetter and several using the Royal Purple.
I am a firm believer in the products - Redline works great for me and have heard good reports about the other. I use it in my dirt bikes, boat, 2 cars and 2 trucks. I was having problems in my Miata and dirt bikes with the summer heat and not getting good cooling with even with large aftermarket aluminum radiators when not moving at a good clip - the Water Wetter dropped temps over 10 degrees. I can really tell the difference in traffic in the Miata, keeps the engine from getting extremely hot and the valves from rattling. The dirt bikes seem to do better on really hot days too.