Transmission oils are a special class of machine oils intended for use in various friction units of vehicle transmissions, as well as in a variety of gearboxes. Motor and transmission oils are produced on a mineral or synthetic basis, and then alloyed with a variety of additives, but the requirements for transmission are higher, since they work at much higher loads and sliding speeds of rubbing surfaces.
One of the most important indicators of such oils is the ability to create a
protective film on the surface of friction units to prevent mechanical contact between them. In
hypoid gears, oils are used, which, when entering into a chemical reaction with the surfaces of the parts, prevent them from boiling up and possess extreme pressure properties. During operation, transmission oils are oxidized and contaminated, so they must be changed periodically. An oil with a low oxidation tendency is considered good. Since different manufacturers use different additives, you can not mix their products. When replacing the transmission unit, rinse thoroughly with the same fresh oil, if possible.
Transmission oils are classified either by viscosity or by performance properties. So according to SAE (Association of Automotive Engineers) they are divided into nine classes - five summer and four winter (with index W). For each class, a viscosity index is indicated. SAE 70W or SAE 250, for example. For all-season oils, a double index is indicated (SAE 80W-90, etc.).
API (American Petroleum Institute) divides oils into 6 classes, depending on their operational properties - GL-1 ... GL-6. These classes are similar to those used in Soviet GOST 17479.2-85 TM1 ... TM6. The fifth and sixth groups are used for hypoid gears and have the highest lubricating and extreme pressure ratings.
Choosing gear oils, you should focus primarily on the conditions of their work. If the class is lower than necessary, the mechanism will simply fail, and a higher class of oil has a higher cost, which will lead to cost overruns. It is also important to consider at what temperatures the transmission will work, and based on this, select a lubricant. SAE 85W, for example, will operate at temperatures down to -12 ° C, and 85W already up to -40 ° C. Automatic transmissions use a special low-viscosity fluid, so neither motor nor transmission oils are used there.
Today in the market a large range of manufacturers offer their gear oils - Lukoil, Total, Texaco, Rexol, Norsi and many others. The variety of additives used determines both the level of quality of oils and their price range. Mineral, usually costs less, but serves less, synthetics are exactly the opposite. A good compromise in this case would be semisynthetics, like the middle ground between the first two. You should also be careful about fakes and not buy oil in unverified stores. Changing a box is always more expensive than taking a good, high-quality oil, especially since now they nursing from 60 thousand or more kilometers from replacement to replacement.