Extreme summer heat can cause motor oil to break faster and lose viscosity. Amsoil was tested by an independent laboratory and ran at 304 degrees F for 90 minutes. Afterwards the oil was put back into the engine and the test run again. After running the test a second time the Signature Amsoil still maintained it’s viscosity and delivered 40% cleaner pistons that required by industry standards. Amsoil Signature series oils have been tested in the field and labs at temperatures far beyond those your engine will encounter and still provided superior protection!