Can I use 75w90 gear oil instead of 75W85?
In short, you should not assume it's okay to substitute; only do so if the manufacturer explicitly allows it. Otherwise, use the exact specification.
Gear oil grades like 75W-85 and 75W-90 describe how the oil behaves at cold start and at operating temperature. While the two grades are similar, the difference in high-temperature viscosity can affect shifting feel, lubrication, and reliability in certain transmissions. Always start with the vehicle's owner manual or service bulletin to confirm acceptable alternatives.
Understanding the grade numbers
Gear oil grades such as 75W-85 and 75W-90 indicate both cold-temperature performance (the "W" rating) and high-temperature viscosity (the second number). The first part (75W) means the oil flows reasonably well in cold weather. The second part (85 or 90) indicates how thick the oil remains at 100°C. The higher the second number, the thicker the oil will be when hot, which can influence how gears mesh and how well lubrication is maintained under load.
Because the two numbers are close, many transmissions tolerate either grade, but not all do. The difference is typically modest but can matter for shifting feel, gear engagement, and bearing lubrication, especially in transmissions engineered for a specific viscosity.
- Cold-flow behavior is similar since both are 75W, so starting in cold weather should feel similar.
- High-temperature viscosity for 75W-90 is typically a bit higher, meaning the oil is thicker when hot, which can affect gear engagement in some units.
- The oil's additive package (friction modifiers, anti-wear agents, and LSD modifiers) must meet the OEM spec; some 75W-90 oils include friction modifiers that can cause chatter in certain limited-slip differentials.
- Some transmissions are specified for GL-4 oils, others for GL-5 (or a GL-4/GL-5 blend); ensure the oil you choose meets the required specification, not just the viscosity grade.
In practice, the ability to substitute depends on the vehicle's specification. Always check the owner's manual or a dealership for guidance to avoid improper lubrication or warranty issues.
When substitution might be acceptable
There are situations where substituting 75W-90 for 75W-85 may be permissible, but only if the manufacturer explicitly allows it or if a service bulletin provides approval. If you are in a scenario where the recommended oil is not available, you may consider alternatives only after verifying compatibility with your transmission's design and any LSD components.
Conditions that might support substitution include:
- The owner's manual or dealer-approved service bulletin lists 75W-90 as an acceptable alternative to 75W-85.
- The vehicle operates in an environment where high-temperature stress is not excessive, and the slightly thicker high-temperature oil will not impede lubrication or shifting.
- You are performing a top-up or fill in between scheduled service intervals and the brand of 75W-85 is temporarily unavailable, with a clear manufacturer-approved alternative of 75W-90.
When substitution is permitted, follow the OEM's guidelines for quantity, service interval, and any special friction-modifier considerations.
Practical steps if you decide to use 75W-90
If you can confirm approval from the manufacturer or service bulletin, here are practical steps to minimize risk:
- Choose a 75W-90 gear oil that carries the correct API/GL rating required by your transmission (for example, GL-4 or GL-5 as specified).
- Avoid oils with friction modifiers that are not compatible with your LSD (if applicable). Some LSDs require specific friction modifiers or no modifiers at all.
- Ensure the oil meets the OEM's viscosity target and performance standards; use reputable brands or OEM-recommended equivalents.
- Record the change and monitor shifting behavior; if you notice noise, hard shifts, or slipping, revert to the recommended grade as soon as possible.
Note that using a non-specified oil may affect warranty coverage or long-term reliability if issues arise, so proceed with caution and obtain official guidance when in doubt.
Summary
The direct answer is: not automatically; you should only substitute 75W-90 for 75W-85 if the vehicle's manufacturer explicitly allows it. The main differences lie in high-temperature viscosity and the potential interaction with friction modifiers used for limited-slip differentials. Always verify the OEM specification in the owner's manual or service bulletin, and choose an oil that matches the required viscosity and GL rating. If in doubt, consult a qualified mechanic or dealership before changing your transmission oil.
What is the difference between 75W and 85W gear oil?
Viscosity Ratings: 75W-85 Gear Oil: The "75W" indicates the oil's viscosity at lower temperatures (Winter), meaning it flows more easily in cold conditions. The "85" represents its viscosity at operating temperature.
Can you mix 75W80 and 75W90?
75W90 offers better shear stability than 75W80 and can generally withstand higher loads in the transmission. Oils of different SAE classes can be mixed with each other.
What is the meaning of 75W85?
SAE 75W85 meaning
The value before the "W" (W = winter suitability) indicates the flowability of the oil at cold temperatures. Transmission oils with 75W remain flowable down to -40°C. The number "85" refers to the kinematic viscosity of the transmission oil at 100°C. This ranges from 11.0 - 13.4 mm²/s.
What happens if you use thicker gear oil?
It is very important to choose the right oil viscosity because if it's too thick, it could cause slipping gears and rough shifting, and if it's too thin, Then the gears will slip and shift roughly.
