Transmission and Differential Gear Oils

Many lube oil brands are readily available for use in truck manual-gearshift transmissions. Transmission oils specified by an OEM are categorized by the American Petroleum Institute (API) and the Society of Automotive Engineers (SAE) classifies the viscosity of lubricants. A "W" appearing after the oil viscosity grade (15W-40) indicates that the product has met requirements for winter use.

Transmission SAE viscosity grade requirements can vary between SAE 30 and SAE 90 based upon the ambient temperature of the transmission oil. The following table lists the oil recommendations for the heavy duty Rockwell twin-countershaft transmissions. These same lube oil specifications are adopted by Eaton/Fuller, Spicer, Mack, Volvo, GMC, and ZF.

Lubricant Type Grade (SAE) Outside Temperature
Heavy Duty Engine Oil
Above 10 deg F
MIL-L-2104B, C, or D
or API-SF, -SG, -CD or -CE


Above 10 deg F
Below 10 deg F
Mineral gear oil with rust and
oxidation inhibitor (API-GL-1)*
Above 10 deg F
Below 10 deg F
Synthetic oil, Rockwell spec. 0-81*
*Multiweight and EP gear oils are not recommended. Do not mix oils in the transmission.

Synthetic Lubricants

Synthetic lubricants have such great advantages that they are used in all jet aircraft and space vehicles. These same advantages are the basis for use by many transmission manufacturers. The CD50 classification is widely accepted by Eaton/Fuller, Spicer, and Rockwell products. Synthetic oils have been chosen as lubricants of choice for transmissions and differentials. Some OEMs fill this equipment with these oils at the factory. Some OEMs also offer a longer warranty if the proper synthetic lubricant is used instead of mineral based oils.

Most people have heard that synthetics are better than petroleum based lubricants but they don't know why. Well, here's why. Because synthetic oil is composed of molecules that are uniform in weight and shape, its heat of vaporization is much higher (more than 300C compared to conventional oil which evaporates at temperatures as low as 175C). This means that you will burn less oil and have less sludge, which is the result of evaporated oil. Added slipperiness is another positive attribute of synthetics. The uniform length of synthetic oil polymers allows them to more easily slide over one another. Mineral-based oil has a film strength of about 500 psi, synthetics are closer to 3000 psi. This means that the oil is less likely to be pushed out from between two metal surfaces where a lot of pressure exists, like bearings and gears. The advantages of synthetics are so great that they out weigh the initial high cost.

A synthetic lubricant will sell for 2 to 6 times the price of petroleum based oil. The synthetic lubricants resist oxidation and will not create varnish and sludge at high temperatures. This feature is important in the newer truck designs where the transmissions are smaller (can dissipate less heat), where aerodynamics reduces airflow over the transmission, and where the high torque E-engines increase the loading and torsional forces on transmission gearing. In cold weather operation, the synthetic lubricant provides better low temperature fluidity to improve startup protection. Synthetics also posess improved wear protection and oil film strength, and extended drain intervals due to the oil's resistance to thermal breakdown.

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