This paper reviews a series of durability field performance tests that resulted in the development of an all-synthetic automotive SAE 75W-90 gear lubricant. The synthetic gear lubricant meets the requirements of API GL-5 and MIL-L-2105C. The synthetic gear oil was field tested in heavy-duty truck axles including Rockwell SQHD and SQHP, Eaton DS-380, and Mack SW-34D models. In general, lubricant drain intervals were extended as high as 250,000 miles with parts inspections carried out at 100,000 to 500,000 miles. All axles inspected indicated that satisfactory lubrication was achieved. The parts analysis and oil inspection analysis are included in this paper.