The Importance of Early Gear Lubricant Replacement for Long Differential Life

Unbeknownst to many motorists, most differential wear occurs during the break-in period. 

Motorists know when they’re supposed to change their motor oil.  They have oil life monitors, oil change centers and commercials all telling them when it’s time for an oil change.  Differential oil changes, on the other hand, often get overlooked.  Many people don’t even think of the differential when performing routine maintenance on their vehicles and don’t realize four-wheel drive trucks have two differentials and a transfer case that all require service.  In fact, according to one quick lube company, only one to two percent of their customers purchase a differential gear lube change.

Most differential wear occurs during the break-in period.  Because differentials are not equipped with filters, break-in metals are suspended in the oil, causing increased wear as the particles mesh between the gears.  Hauling heavy loads and towing heavy trailers cause additional stress to the differential during the break-in period and can cause premature differential damage or failure.  Changing the gear lube after the break-in period (about 3,000 miles) is a low-cost maintenance investment that provides a significant payoff, including greatly reduced wear, extended differential gear and bearing life and protection for expensive vehicle investments.  Auto manufacturers recognize the importance of draining abrasive break-in materials.  As seen in Chart 1, some manufacturers recommend an initial drain interval of between 500 and 3,000 miles.

Differential internal components consist of six gears (one pinion, one ring, two side and two spider gears), six bearings (two pinion, two carrier and two axle) and sometimes include a clutch setup for limited slip performance.  All of these parts require high quality, clean gear oil in order to perform at an optimal level.  Most pickup trucks, SUVs and vans operate in severe service conditions, including towing, hauling, steep hill driving, commercial use, plowing, racing, off-road use, rapid acceleration, frequent stop and-go operation and high ambient temperatures.  These severe service operating conditions subject the differential to extreme pressures and operating temperatures.

New vehicles such as turbo diesel trucks and vehicles with V-10 engines boast more horsepower and torque than their predecessors, but differential designs have remained virtually unchanged.  Differentials today are subjected to severe duty service and encounter more stress and heat than was seen only a few years ago.  Modern gear oils are faced with the challenge of providing adequate wear protection during severe service operating conditions, while also providing maximum fuel efficiency.

In fact, according to a 2005 SAE paper entitled Breaking the Viscosity Paradigm: Formulating Approaches for Optimizing Efficiency and Vehicle Life, “Concurrent with the strong drive toward better fuel economy, consumers have been demanding increased performance, which has required axle lubricants with enhanced durability protection and lower operating temperatures.  There has been a 34% increase in engine horsepower over the last decade, while axle gear sizes have remained constant, sump capacities have been lowered, and drain intervals extended.  In the light truck segment there has been a 93% horsepower increase since 1981.”

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, California 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 230 degrees F in the new axle and 203 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 a lubricant’s inability to control friction and increased heat under high stress conditions.

AMSOIL SEVERE GEAR™ Synthetic Gear Lubes are recommended for turbo diesel pick-ups, SUVs, vans, delivery/utility vehicles, light, medium and heavy-duty trucks, buses, heavy equipment, 4x4s, tow trucks, race cars, tractors and motor homes.

AMSOIL Synthetic Gear Lubes save motorists money through extended drain intervals, and they also cost less per quart compared to many competing OEM gear lubes.

A test on four separate vehicles further demonstrated the importance of changing the factory-fill gear lube within the first few thousand miles.  Oil analysis results revealed most of the wear in vehicle one occurred within the first 6,869 miles.  Despite higher mileages, vehicles two and three only showed slightly higher iron wear than vehicle one.  Vehicle four was the only vehicle which had the factory-fill oil changed to AMSOIL synthetic gear lube, and despite significantly higher mileage than the first three vehicles, it showed a significantly lower level of wear. 

The chart (right) and photos below provide visual evidence of the wear materials present in the differentials of each vehicle.  The photos clearly show that the heaviest amount of wear occurs early in the break-in period.  Wear is significantly reduced when the gear lube is changed to AMSOIL synthetic gear lube following the break-in period.