Four GL-5 gear lubricants, one containing a dispersed solid borate additive, two sulfur-phosphorus (S-P), and a S-P synthetic, were evaluated in the axles and transmissions of 26 new Class 8 trucks in a 250,000-mile field test. Significantly lower axle break-in wear (factory to first oil change) was found for the borate lubricant versus a S-P factory-fill oil. The four test oils showed statistically significant differences in axle wear as shown by iron wear plots and analysis of end-of-test parts: the borate and synthetic oils were equal while the other two oils gave higher wear. All oils gave acceptable seal performance. The borate lubricant showed the best thermal stability as judged by parts cleanliness.