Heavy-duty axle and manual transmission lubricants formulated with Group II base oil and borate additive chemistry were tested in a 250,000-mile, extended-drain, on-highway field test. For reference, units using a synthetic sulfur-phosphorus axle lubricant and a synthetic engine oil type transmission fluid were monitored. The field test involved two major fleets with approximately 200 trucks. Twenty axles and transmissions using the Group II borate chemistry lubricants were inspected as they reached greater than 275,000 miles. Oil analysis was performed on the units running on the test and reference lubricants. Test data indicated that all four lubricants performed well. However, end-of-test oil analysis data showed significantly less wear with the Group II products formulated with borate chemistry compared to the reference products. In the axle test, the iron wear rate for the Group II borate chemistry lubricant was 56% lower than the iron wear rate for the synthetic axle lubricant. In the transmission test, the iron wear rate for the Group II borate chemistry lubricant was 150% lower than the iron wear rate for the synthetic transmission lubricant. The Group II borate additive chemistry test fleets exceeded 27.5 million miles of proven performance with a reliability rating of 99.6% at 250,000 miles.