Transmission Oils: How Are They Classified and When Should They Be Changed?
Transmission oils play a very important role in motor vehicles. Not only do they lubricate the components of the transmission — especially the bearings and gears — they also dissipate heat and protect surfaces from rapid wear. However, as with all oils, over time they lose their optimal properties and need to be replaced.
In the following lines, we’ll advise you on how to choose the right transmission oil and explain the differences between the types for manual and automatic transmissions.
Oil for Manual Transmission
Oils for manual transmissions are specific because they have high viscosity and usually contain organic sulfur compounds. They can withstand extremely high temperatures and create a durable oil film that won’t break even under heavy loads.
Transmission Oils and Their Viscosity
As with engine oils, the viscosity of transmission oils is evaluated according to SAE specifications. There are 4 winter grades (70W, 75W, 80W, 85W) and 5 summer grades (80, 85, 90, 140, 250). The lower the winter-grade number, the lower the viscosity and the better the oil performance at sub-zero temperatures. The higher the summer-grade number, the better its resistance to extremely high temperatures.
Performance Grades of Transmission Oils
Manual transmission oils are divided into 3 performance grades under the API classification:
• GL-4: These lubricants are suitable for transmissions with spiral bevel gears or hypoid gears operating under light to moderately heavy conditions. Designed for regular passenger cars, light trucks, as well as tractors and garden machinery.
• GL-5: Oils of this grade are mainly intended for hypoid gear transmissions and differentials working under more demanding conditions with shock loads and high speeds. However, due to phosphorus and sulfur additives, they can cause synchronizer wear in manual transmissions, so they must only be used if the manufacturer explicitly allows it.
• MT-1: This oil type is for nonsynchronized manual transmissions in buses and heavy trucks under demanding conditions. It provides excellent thermal and oxidative stability as well as enhanced protection for gears and bearings.
Manufacturer Standards
Just like with engine oils, there are various OEM standards for transmission oils. The most well-known are standards from ZF, MERCEDES-BENZ, CATERPILLAR, MAN, US MIL, and SCANIA — for example, MB 235.7 (oils for differentials) or MB 235.8 (oils for hypoid gears).
Oil for Automatic Transmission
Automatic transmissions have a more complex design than manual ones and usually feature many small channels through which the oil circulates. That’s why it’s important to use a low-viscosity oil. Automatic transmission fluid generally has a lower viscosity and a lower boiling point than manual transmission oil.
Automatic Transmission Fluid — often abbreviated as ATF — usually contains detergents to reduce sludge formation, anti-foam agents, and other additives to extend the lifespan of the transmission.
Transmission Oil Specifications
When choosing an oil for an automatic transmission, follow the manufacturer’s recommendations. Every OEM has its own set of specifications applicable to specific types of transmissions. Common ones include DEXRON (GM), MERCEDES-BENZ, ZF, FORD, ALLISON, CATERPILLAR, BMW, or VW.
For example, the ATF DEXRON VI specification introduced in 2005 is for new 6-speed GM and Ford transmissions, while MB 236.20 is for CVT transmissions in MERCEDES-BENZ vehicles.
When Should Transmission Oils Be Changed?
For modern manual transmissions, manufacturers usually recommend changing the transmission oil every 5–7 years or every 80,000–120,000 km. In the past, the so-called “lifetime fills” were common, and oil replacement was not recommended. The old oil would compensate for minor tolerances and leaks that new oil could wash away — potentially damaging the transmission.
For automatic transmissions, it’s ideal to change the fluid every 70,000–120,000 km according to the manufacturer’s recommendations. When selecting transmission oil, always follow the OEM specification and avoid mixing different types.
