I would speculate that there are many variables that could affect the readings you obtained. Just out of curiosity, what kind of oil were you using before you switched to Amsoil? The reason I ask is the factory specified Mobil 1 4T is very close to Amsoil in most test results (I used to be an Amsoil dealer, very familiar with their tests). If however you were running a Dino oil, that could account for some of the difference.
As I said, monitoring the ECU reported temperatures, I could see virtually no change in the different fills. But, there's certainly nothing wrong with Engine Ice. Here in Arizona its a very popular alternative. My dealer seemed to use it for all rad fluid changes.