To avoid equipment failure, a gear oil should remain a homogeneous liquid and the performance-enhancing additives should not separate out when the oil is stored for an extended period of time.

In addition, because different oils are often mixed when topping off, gear oils from different manufacturers, or containing different base fluids or performance-enhancing additives should be completely miscible and compatible with each other. Any incompatibility of such mixtures may also result in equipment failure if gelation or additive dropout occurs.

The test procedures described in this test method are designed to evaluate the performance of gear oils in each of the above circumstances.

This test method is based on the separate test methods FED-STD-791/3440.1 and FED-STD-791/3440.2. Minor changes have been made to the FED test methods to provide a coherent unified procedure. These changes do not significantly alter the test procedures. This test method has, therefore, potential for use as an alternative to the FED test methods in gear oil specifications such as SAE J2360 and Specification D5760.