Standards and Specifications for Mechanical Transmission Oils.
Any moving mechanism in a car requires reliable lubrication — the engine, transmission, individual components of the suspension and braking systems, etc. The only exceptions are components that operate on a friction basis: brakes, clutches. The wide variety of design solutions, technologies, materials, and operating conditions has led to the emergence of a fairly broad range of lubricants that seemingly serve the same function.
The differences between various transmission oils are no less significant than those between motor oils, and choosing the right product for a manual gearbox or reducer should be approached with equal responsibility. Despite the large number of proprietary approvals from car manufacturers, there are three main criteria by which all transmission oils for manual transmissions, transfer boxes, and bridges are distinguished: quality class API GL, quality class according to the ZF system, and viscosity according to the SAE system. Different-named manufacturer approvals often overlap regarding the same products and ultimately reduce to API GL, SAE, and ZF. Exceptions do occur, but they are extremely rare and usually result from a manufacturer's proprietary formulation.
Classification of Oils for Manual Transmissions and Bridges
According to the API GL classification system, transmission oils are divided into 6 quality classes based on their intended purpose and the number of anti-wear and multifunctional additives.
Oils of classes API GL-1, GL-2, and GL-6 are no longer used today. GL-1 and GL-2 due to outdated properties, and GL-6 because their use is neither technologically nor economically justified.
Oils of class GL-3 are intended for medium-loaded conical and similar gears of trucks and synchronized manual transmissions of passenger cars, containing up to 2.7% anti-wear additives. Oils of this class are not suitable for hypoid gears.
GL-4 oils are probably the most universal of all, as their use is stipulated for conical and hypoid gears, synchronized and non-synchronized transmissions operating in various conditions. GL-4 oils of different viscosities are the primary choice for use in manual and robotic gearboxes and contain up to 4.0% anti-wear and anti-scuff additives. Typically, when referring to 'oil in the gearbox' or 'oil in the box', it is GL-4 oil that is meant.
Class GL-5 was developed for use in extremely loaded gears operating under harsh conditions. Oils of this class contain up to 6.5% anti-wear, anti-scuff, and other multifunctional additives. Their primary application is in hypoid gears with significant axis displacement and other transmission components. In synchronized manual transmissions, GL-5 oils are used quite rarely due to the negative impact of the increased number of anti-wear additives on the operation and lifespan of synchronizers. GL-5 oils with a special LS additive package are also used in high-friction differentials.
Labeling and Viscosity of Transmission Oils
The definition and labeling of the viscosity of transmission oil follows the same principle as motor oil labeling and is determined by the SAE J306 standard, according to which the labeling is in the format yyW-xx, where 'yy' represents 'winter' viscosity, and 'xx' is the viscosity of the oil at 100 degrees. Depending on the operating temperatures and loads, satisfactory performance can also be achieved with only 'winter series' oils. For example, the original oil in the VW AG gearbox has a viscosity of 75W.
The ZF labeling system has developed 15 types of transmission oils, denoted by codes from ZF TE-ML-01 to ZF TE-ML-15, which include units from the simplest non-synchronized transmissions with constant mesh gears to marine transmissions and complex hydraulic systems for special and military vehicles. Each type of transmission oil identified by a ZF code specifies its intended use, viscosity classes, and permitted products. The ZF classification, along with the proprietary approvals from car manufacturers, is fundamental for European developers and manufacturers of transmissions and vehicles.
How and Which Oil to Choose for the Gearbox, Transfer Case, or Bridge?
First and foremost, it is essential to learn the recommendations from the car manufacturer regarding quality classes API GL or ZF and viscosity according to the SAE standard. A fairly universal and generally the most common choice for manual synchronized transmissions is oil of class GL-4 with viscosity 75w-80 – 75w-90, and for transfer cases and reducers – GL-5 with viscosity 75w-90. However, there are always alternatives.
How often should the oil in the gearbox be changed, and is it worth doing?
No matter how high-quality the oil is and how finely processed the transmission parts are, wear products inevitably appear during operation, and the oil oxidizes over time. While car manufacturers often do not specify intervals for oil changes in gearboxes or bridges, claiming that the factory product is designed for the entire service life, the exact terms meant are unknown. Moreover, the oil gradually loses its lubricating and anti-wear properties due to oxidation and mixing with wear products. Hence, if the manufacturer does not provide a replacement schedule, the first oil change in the gearbox should be carried out at 100,000 to 150,000 kilometers of mileage. The general schedule for replacing synthetic transmission oil is 90,000 to 100,000 kilometers, while semi-synthetic oil should not be changed less frequently than every 50,000 to 60,000 kilometers of mileage. In any case, under high loads, exposure to water in gearboxes or bridges, the intervals should be shortened.
