A GL-5 gear lubricant containing a dispersed solid borate additive and a CD synthetic engine lubricant were evaluated in the overdrive transmissions of 17 new Class 8 trucks in a 500,000-mile per truck, on-highway field test. With 500,000-mile oil drain intervals, both lubricants gave acceptable performance as judged by end-of-test inspections: deposit/sludge formation was minimal, overall wear was low, and seal/yoke performance was good. The borate lubricant gave a statistically significant lower wear rate than the synthetic lubricant based upon oil sample analysis; inspections also showed lower wear on some parts. Additionally, oil sample analysis from the tandem drive axles showed that the forward axle gave a statistically significant higher wear rate than the rear axle. Transmission and drive axle oil temperatures were monitored during a portion of the test.