Water works just fine for a leak test and to see if the thermostat is good. Just drain it out when you are done. It will freeze. Water is my choice just because it won't hurt the environment if it does leak and...it is cheap. But don't forget to drain it out when the test run is over. Antifreeze will help protect the water pump and it it is better at heat transfer than plain water. Also will provide some corrosion protection with the different materials encountered. Did I mention the thing about freezing? Yes use water just don't forget to drain and replace. smile

Steve H