1. The core role and classification of gear oil
1. Core function
Gear oil is a "protective agent" for gear transmission mechanisms such as gearboxes and differentials. It mainly ensures the operation of components through three mechanisms:

When the gears are meshed under high load, a strong oil film is formed to reduce metal friction and wear, especially for high-precision components such as hyperbolic gears;
Quickly remove the heat generated by the operation of the gears to avoid deformation of parts or oxidation of oil due to high temperature;
Isolate moisture and air to prevent metal rust, while buffering the impact noise of gear meshing and improving driving smoothness.

2. Detailed explanation of classification standards

Viscosity grade (SAE): For example, in "75W-80", "W" represents winter applicability. The smaller the number, the better the low-temperature fluidity (75W is more suitable for severe cold areas than 80W); the latter number represents high-temperature viscosity. The larger the number, the higher the oil film strength (such as 140 is suitable for heavy-load conditions).
Performance grade (API):
GL-4 is suitable for manual transmissions and ordinary spiral bevel gear differentials, and is resistant to medium-load wear;
GL-5 adds more extreme pressure additives and is designed for hyperbolic gears and heavy-load transmissions, with stronger anti-wear performance, but it should be noted that if the manual transmission contains copper synchronizers, misuse of GL-5 may cause corrosion.
2. Key points for selecting gear oils for different models
1. Manual transmissions for family cars
Preferably choose API GL-4 grade, with a viscosity of 75W-80 or 80W-90, and avoid using GL-5 grade oil to prevent corrosion of copper parts. For example, for the Volkswagen MQ250 transmission, special oils such as Volkswagen Titan 75W-90 are recommended.

2. Rear axle differential (especially four-wheel drive vehicles)
General models can use GL-5 grade 80W-90; if it is a hyperbolic gear or with limited slip function (LSD), you need to choose GL-5 grade oil containing specific additives (such as Toyota Prado rear axle needs to add LSD additives), the viscosity can be adjusted according to the regional temperature, 75W-140 in the north, 85W-140 in the south.

3. Dual Clutch Transmission (DCT)
The manufacturer's specified oil must be used, such as Volkswagen DQ series transmission needs G052 171 A2, ordinary gear oil will cause gear shifting or clutch overheating, the oil change cycle is usually 40,000-60,000 kilometers.

4. Heavy vehicles
For truck transmissions or engineering vehicle gear systems, high-viscosity GL-5 grade oil (such as 85W-250) must be used. Synthetic oil can extend the oil change cycle to more than 100,000 kilometers, while taking into account extreme pressure resistance.
3. Differences between synthetic oil and mineral oil and applicable scenarios
The advantages of synthetic gear oil are strong high-temperature stability, good low-temperature fluidity, and a service life of 60,000-100,000 kilometers (more than twice that of mineral oil). It has better extreme pressure resistance and is suitable for high-performance vehicles, four-wheel drive off-road vehicles or vehicles with frequent heavy loads, but the price is relatively high (about 2-3 times that of mineral oil).

Mineral gear oil has low cost and wide compatibility, and is suitable for light-load gear systems of ordinary household cars, but it is easy to oxidize at high temperatures. The oil change cycle is usually 30,000-50,000 kilometers, and there may be insufficient fluidity in winter in low-temperature areas.

4. Oil change cycle and abnormal state judgment

1. Conventional replacement recommendations

Mineral oil: Replace every 30,000-50,000 kilometers or 2 years, whichever comes first;
Synthetic oil: It can be extended to 60,000-100,000 kilometers or 3-4 years, but it needs to be shortened to 50% of the cycle for frequent off-road, mountainous driving or high-temperature environments (for example, synthetic oil is recommended to be replaced every 50,000 kilometers under heavy load conditions).

2. Identification of abnormal oil

Black color and pungent burning smell may be caused by high temperature oxidation or metal debris contamination. Check the gear wear immediately;
Oil stains appear at the joints of the gearbox or differential housing. It may be due to aging of the oil seal or damage of the gasket. Timely maintenance is required to avoid oil shortage;
The aggravation of the gear shift and the increase of abnormal noise during driving may be due to insufficient oil viscosity or reduced anti-wear performance. The oil status needs to be tested and replaced.

V. Common misunderstandings in purchasing and use

Misunderstanding 1: GL-5 grade oil is "universal"
If the manual transmission is equipped with a copper synchronizer (such as most family cars), GL-4 grade oil must be used. The sulfur and phosphorus additives of GL-5 will corrode copper parts and cause difficulty in shifting.

Misunderstanding 2: The higher the viscosity, the stronger the protection
High-viscosity oil has poor fluidity at low temperatures. In the northern winter, it may cause "dry friction" when the gear is cold started. It is recommended to choose according to the local minimum temperature. For example, in areas below -20℃, 75W-90 is preferred instead of 85W-140.
Misconception 3: Mixing different brands of oil
Different brands have different additive formulas (such as differences in anti-wear agents and rust inhibitors). Mixing may produce precipitation and affect the performance of the oil. If you need to change brands, it is recommended to thoroughly clean the gearbox before injecting new oil.
VI. Reference for special oils for typical models
Volkswagen MQ series manual transmission: Volkswagen Titan Super Gear Oil 75W-90 (GL-4 grade);
Porsche PDK dual-clutch transmission: original PDK FLUID special oil, oil change cycle 40,000 kilometers, it is strictly forbidden to mix with other oils;
Mercedes-Benz rear axle differential (with LSD): synthetic gear oil that meets the MB 236.14 standard must be used, with a viscosity of 75W-140;
Pickup/SUV heavy-duty rear axle: Mobil 1 synthetic gear oil 75W-140 (GL-5 grade), suitable for high-load conditions of frequent off-road.
Key reminder
The selection of gear oil should be based on the vehicle manual or manufacturer's technical specifications. If you are not sure, you can check the 4S store data through the vehicle VIN code, or directly check the original factory markings on the gearbox/differential housing. Incorrect oil use may cause excessive wear of gears, gearbox failure, and even affect the life of the vehicle's transmission system. It is recommended to give priority to original factory specifications or certified third-party products during maintenance.