The ASTM D6121 (L-37) is a key hypoid gear lubricant durability test for ASTM D7450-08 (API Category GL-5) and the higher performance level SAE J2360. It is defined as the ‘Standard Test Method for Evaluation of Load-Carrying Capacity of Lubricants Under Conditions of Low Speed and High Torque Used for Final Hypoid Drive Axles’. Pass/fail is determined upon completion of the test by rating the pinion and ring gears for several types of surface distress, including wear, rippling, ridging, pitting, spalling and scoring. Passing the L-37 in addition to the other tests required for API Category GL-5 credentials, as well as the more strenuous SAE J2360 certification, requires in-depth formulating knowledge to appropriately balance the additive chemistry.This paper describes the results of ASTM D6121 experiments run for the purposes of better understanding gear oil durability. Two gear lubricants, differing only in the antiwear chemistry, were used: gear lubricant A was designed to pass the test, while gear lubricant B was intentionally formulated to fail for gear ridging. In this series of experiments: the cooling demand required to maintain a constant oil temperature was monitored continuously; the oil was sampled every two hours for evaluation of changes in oil chemistry and particulate levels; and surface analysis was performed on end-of-test gears not only from tests run to completion, but also from tests stopped early.It will be shown that gear ridging is an abrupt process, characterized by a sharp increase in cooling demand, iron content and particulate levels. Protective phosphate films are formed on the gear surfaces in the earliest stages of both passing and failing tests, but in ridging failures the phosphate is replaced by sulfide films not capable of providing adequate gear tooth protection, which then allows ridging.