Oil Temperature
Further evidence of stress and increased temperatures during the differential break-in period is documented in a 2005 SAE paper entitled The Effect of Heavy Loads on Light Duty Vehicle Axle Operating Temperature. A light duty GM truck towing 14,000 pounds was driven from Orange County, Calif. to the Nevada state line. The test was conducted with both a new axle and a broken-in axle. Over level ground towing, oil temperature was measured at 110 degrees F in the new axle and 95 degrees F in the broken-in axle. Oil temperature over the most grueling portion of the trip, during which a maximum 6% grade was encountered, revealed the new axle was operating at 350 degrees F and the broken-in axle was operating at 300 degrees F. Laboratory dynamometer test results simulating a truck hauling a trailer provided similar results, with level ground towing temperatures recorded at 266 degrees F with the new axle and 194 degrees F with the broken-in axle and towing temperatures (at a 3.5% grade) recorded at 370 degrees F with the new axle and 295 degrees F with the broken-in axle.
AMSOIL Severe Gear 75w-90 and 75w-140 Synthetic Gear Lubes 
are formulated for severe service applications, protecting differential gears for extended drain intervals of up to 50,000 miles in severe service and 100,000 miles in normal service, or longer where specified by the vehicle manufacturer. Formulated with shear stable synthetic base stocks and an extra treatment of additives, Severe Gear, Gear Lubes provide unsurpassed wear protection and friction reduction, while their excellent thermal stability prevents thermal runaway, a phenomenon caused by lubricant’s inability to control friction and increased heat under high stress conditions.