What gear oil do I need for my differential?
The gear oil you need is the one specified by your vehicle’s manufacturer. In practice, most passenger-car differentials use a 75W-90 or 80W-90 GL-5 oil, while heavy-duty axles may require 75W-140 or 85W-140. Always consult your owner's manual or axle code to confirm the exact grade, viscosity, and whether a friction-modifier is required.
Differentials vary by design—open, limited-slip, or locker—and by climate and duty cycle. The oil’s job is to lubricate gears under load, control temperature, and protect against wear and corrosion. Picking the wrong oil can lead to quieter operation, faster wear, or overheating. This article explains how to determine the right gear oil for your differential and how to maintain it properly.
Common grades and what they mean
Here's a quick reference to the viscosity grades you’re likely to see, along with typical usage. Always verify with your model’s specification.
- 75W-90 GL-5: The most common modern choice for hypoid differentials in passenger cars and light trucks; good all-temperature performance; available as conventional or synthetic.
- 80W-90 GL-5: An alternative where 75W-90 isn’t specified; still widely used in many diffs and markets.
- 75W-140 GL-5 or 80W-140 GL-5: Higher-viscosity option for heavy loads, towing, hot climates, or high-heat operation; often used in trucks or LSD-equipped axles.
- 85W-140 GL-5: Used in some heavy-duty applications; less common in modern passenger cars.
- GL-4 (and GL-4-compatible oils): Some older diffs or non-hypoid setups historically required GL-4; many modern products are GL-5 and not compatible with some older hardware—check compatibility.
- Friction modifiers: If your differential is limited-slip (LSD) or uses a clutch-type design, the oil may require friction modifiers or you may need to add modifiers separately per the manufacturer’s guidance.
Note: The exact recommendation varies by model and axle design. Always verify with the owner’s manual or axle tag to confirm grade, viscosity, and whether a friction modifier is needed.
Choosing the right oil for your differential
Before selecting oil, identify whether your differential is open, limited-slip, or locker-equipped, and consider climate, towing, and maintenance plan. The right oil depends on performance needs and component compatibility.
Key considerations include official specification, presence of friction modifiers for LSDs, and whether you prefer synthetic or conventional oil. Synthetic oils typically offer better temperature performance and longer service life, but must still meet the vehicle’s specification.
- Check the owner’s manual or axle tag for the exact viscosity and GL rating (for example, 75W-90 GL-5).
- If the differential is limited-slip, verify whether the oil contains friction modifiers or whether you must add them per the manufacturer’s guidance.
- Climate and use matter: hot climates or heavy towing may benefit from 75W-140 or 85W-140; milder conditions often use 75W-90 or 80W-90.
- Choose between synthetic and conventional based on budget, driving conditions, and the manufacturer’s recommended intervals.
With the correct specification, you’ll reduce wear, control heat, and ensure smooth operation across conditions.
Maintenance: How to drain and refill the differential oil
Replacing differential oil properly involves draining the old fluid, inspecting for metal debris, and refilling to the proper level. The steps below are a common DIY approach; always follow your vehicle’s service manual for torque specs and capacities.
- Warm the vehicle to circulating temperature, then park on a level surface and chock the wheels.
- Raise the vehicle and support securely on jack stands; place a drain pan under the differential.
- If possible, open the fill plug first to ensure you can refill after draining.
- Remove the drain plug and allow the old oil to fully drain; inspect for metal particles or contamination.
- Reinstall the drain plug with a new washer and torque to the specification.
- Remove or loosen the fill plug and begin filling with the correct oil grade until it starts to seep from the fill hole.
- Reinstall the fill plug and torque to spec; wipe away any spills and lower the vehicle.
- Dispose of used oil responsibly and log the service date and mileage for future reference.
Intervals vary by vehicle and oil type. Conventional oil often requires more frequent changes than synthetic oil; follow the manual’s guidance or your maintenance schedule for best results.
Special cases: LSD, all-wheel drive, and older vehicles
Limited-slip and friction modifiers
Limited-slip differentials typically require oils with friction modifiers, or you must add a compatible modifier per the manufacturer’s instructions to prevent clutch chatter and uneven power delivery. Always verify the requirement for your specific LSD setup.
Older vehicles and GL-4 considerations
Some older vehicles may still specify GL-4 gear oil or a GL-4-compatible formulation due to materials used in the gears and seals. Modern GL-5 oils may not be suitable for all older components. Always confirm compatibility with your manual or a trusted service professional.
Summary
Choosing the right gear oil for your differential starts with the manufacturer’s specification. For most modern passenger cars, 75W-90 GL-5 or 80W-90 GL-5 is typical, with 75W-140 or 85W-140 used in heavy-duty or high-heat scenarios. If your diff is limited-slip, ensure the oil includes friction modifiers or add them as directed. For older vehicles, GL-4 compatibility may be necessary. Always consult the owner's manual or axle tag, and follow proper drain-and-refill procedures to maintain performance and longevity of the differential.
