What's the Difference Between 75w80 and 75w90 Gear Oil?
2 Answers
The difference between 75w-90 and 80w-90 lies in their low-temperature viscosity. Both 75w90 and 80w90 are SAE viscosity grades. Transmission oils can be divided into single-grade viscosity types and multi-grade all-weather types, with single-grade viscosity types further categorized into high-temperature and low-temperature types. 75w90 and 80w90 belong to the multi-grade all-weather type. At 100°C, the kinematic viscosity achieved by 75w-90 gear oil is 4.1mm²·s, with a maximum temperature of -40°C to reach 150Pa·s. For 80W-90 gear oil, the kinematic viscosity at 100°C is 7.0mm²·s, with a maximum temperature of -26°C to reach 150Pa·s. The selection of transmission gear oil models depends on the temperature conditions of different regions or seasons. Higher temperatures require gear oils with higher viscosity, while lower temperatures require those with lower viscosity. For medium-duty manual transmissions, 85w/90 gear oil, being a multi-grade all-weather type, offers a wider practical temperature range and can be used year-round. When changing oil, viscosity is the most fundamental selection factor for gear oil. Choosing a viscosity that is too high can lead to overheating of friction surfaces and unnecessary power loss. Conversely, selecting a viscosity that is too low may cause the oil to be thrown off the gear surfaces due to centrifugal force, leading to oil leakage from seals, insufficient lubrication on gear surfaces, increased wear, or even sintering. Therefore, selecting the appropriate viscosity is crucial.
The difference between 75w80 and 75w90 motor oil lies in viscosity. 75w80 is thinner than 90, meaning it flows more easily during cold starts, reducing engine wear and startup difficulties. However, in high-temperature environments such as long summer drives, 75w90 has higher viscosity and can form a thicker oil film, providing better protection, especially suitable for SUVs, pickup trucks, or heavy-duty vehicles. Oil selection should follow the owner's manual recommendations, as newer models or precision engines may perform better with 80 for fuel efficiency, while older vehicles or those frequently driven on mountainous roads may benefit more from 90 for added stability. Seasonal changes should also be considered—using 80 in winter and 90 in summer—and oil change intervals impact engine longevity. Overall, matching the vehicle's needs is key.
