"Duramax" is General Motors' moniker for its diesel truck and van engine. These vehicles are designed for heavy-duty service with the engine and drive-train expected to last several hundred thousand miles. The diesel engine generates a lot of turning force, so good lubrication is essential to prevent gear overheating and failure.
Duramax-equipped vehicles are rear-wheel drive so the differential is in the back of the vehicle. Four-wheel-drive equipped versions have a second differential under the front. A solid axle will not allow the wheels to turn independently of each other, but the gears within the differential make this possible. All of the torque produced by the engine and transmission pass through the differential and on to the rear wheels in two-wheel-drive models. The power distribution is shared with the forward axle in four-wheel-drive versions.
A differential oil must be robust to handle high temperature operation and very long periods between changes. Duramax differential lube relies on the gear action to circulate it without pumps or filters. It also flows toward the wheel through the passage around the axle to lubricate the wheel bearings.
Oil thickens as the temperature lowers and two numbers are generally listed on the container. The oil specified by General Motors for Duramax-equipped differentials is SAE 75W-90. The 75 is the relative viscosity in colder weather, and the 90 is the relative viscosity in warmer weather. The thickness of this oil is much greater than engine oil that might be 10W-30 or 0W-20.
General Motors specifies SAE 75W-90 synthetic oil Part No. 89021677 for the Duramax differential. Synthetic oil is highly refined stock made from crude oil and engineered hydrocarbons produced under strictly controlled conditions. This resulting lubricant lasts longer and provides protection superior to conventional lube.
The Duramax differential does not require frequent checks or changes. The General Motors manual recommends changing the transfer case fluid every 50,000 miles, and the eight quarts of differential lube should be changed at the same time. This interval applies especially if the vehicle is driven under severe conditions such as hauling heavy loads, in extreme temperatures or frequent stop-and-go conditions.
