Gear Oils vs. Transmission Multi-Grade Oils

Authored by: Allan Zieser, STLE, CLS


Gear oils have been multi-grade oils for a long time now. We have gotten used to seeing viscosity grades such as 80W-90, 75W-90, etc. Transmission oils have normally been listed simply by names or specifications such as Mercon V, Dexron VI, etc. They were single-grade transmission oils designed to meet the specifications of various transmission makers. These oils are still with us but now we have newer oils for many of the Automated Manual Transmissions that are on the market. Some of these newer oils spec out multi-grade oils for their lubricants. Multi-grades such as 75W-80, 75W-90 are now common for these newer transmissions.

Does the presence of these newer multi-grade oils mean that we can use one oil for both Transmission and Differentials? No, it does not. These new transmission oils may have similar or the same multi-grade listings on them, but they are vastly different from one another. Mixing up gear oils and transmission oils is a recipe for disaster.

Gear oils for automotive and heavy-duty vehicle applications have a very large amount of Extreme Pressure (EP) additives in them. They are designed for the high wear conditions that exist in differentials and drive gears. These EP additives are often made from corrosive materials that are detrimental to yellow metals (brass, bronze, copper, etc.). While differentials and rear end gear sets do not contain these yellow metals, transmissions often do.

Transmission oils, on the other hand, do not contain these corrosive additive packages and rely on additives such as Zinc and others to accomplish the wear protection they need. Plus, they often have other more specialized additives to protect the transmission and ensure smooth shifting operations.

As stated earlier, putting these new transmission oils in your differentials is a recipe for disaster. The transmission oils will not have the proper additives to handle the loads of the gears, leading to heavy amounts of gear wear and failure. If we put the gear oils into the transmissions, the heavy additives in them will work to destroy these expensive transmissions.

So, how do we prevent mixing up the two? Training your personnel on these oils and the dangers of misuse is of the most importance. Making sure that they understand the differences is vital to a successful maintenance program. In addition to education, take precautionary steps like storing them in a different area of the shop as well as marking them with the words Gear Oil and Transmission Oil to prevent mishaps.



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