What oil should I use in my rear differential?
In most rear differentials, use a gear oil that meets your vehicle’s OEM specification—typically GL-5 gear oil in a viscosity such as 75W-90 or 80W-90, often synthetic. If your car has a limited-slip differential, choose an oil with the appropriate friction modifiers or use the additive recommended by the manufacturer. Always consult your owner’s manual or dealer for the exact spec.
Understanding rear-differential oil specs
Rear differentials require lubricants that protect gears under high loads, resist shearing, and handle heat from continuous operation. The public standards you’ll encounter include API GL-5, and in some cases GL-4. The “weight” (75W-90, 80W-90, 85W-140, etc.) describes how the oil behaves across temperatures and loads. For most modern passenger cars, GL-5 oils in a 75W-90 or 80W-90 range are common, while heavy-duty trucks or high-tow applications may use higher-viscosity options like 85W-140 or 75W-140. If you have a limited-slip differential, you may also need friction modifiers or a manufacturer-recommended additive to ensure smooth operation.
Gear-oil specifications you should know
Key points to verify before purchasing oil:
- Gear-oil family: API GL-5 is the standard for hypoid gears found in most rear differentials; GL-4 is less common for newer vehicles.
- Viscosity: Common choices are 75W-90 and 80W-90; 85W-140 or 75W-140 are used for heavy-duty or high-heat scenarios.
- Synthetic vs mineral: Synthetic formulations often offer better high-temperature stability and cold-temperature flow; some OEMs specify synthetic, while others allow mineral oils.
- LSD compatibility: If you have a limited-slip differential, ensure the oil is compatible with LSD or plan to add the manufacturer’s friction-modifier additive if required.
Choosing the right specification is essential to protecting gears, seals, and bearings over time. When in doubt, rely on the OEM specification printed in your owner's manual or on the fluid-fill sticker under the hood or inside the service manual cover.
Common oil options and when to use them
The following options cover a wide range of vehicles and climates. Use the oil weight and type your manufacturer specifies, and consider climate, towing, and duty cycle when selecting among them.
- 75W-90 synthetic (GL-5): The most widely recommended option for modern passenger cars and many light trucks. Excellent cold-weather flow and high-temperature stability.
- 80W-90 or 85W-140 (GL-5): Traditional choices for older vehicles, heavy-duty use, or hot climates; 85W-140 is common in some trucks and towing applications.
- 75W-140 synthetic: Used in some European and performance models or climates requiring strong film strength at high temperatures; verify LSD compatibility if applicable.
- Mineral 80W-90 or 85W-140: May be specified for older transmissions or certain applications; often cheaper but offers less high-heat stability and longer-term protection than synthetic.
- Notes on GL-4 vs GL-5: Most modern rear differentials require GL-5; GL-4 is less common today and is typically reserved for older gearboxes or specific equipment. Do not substitute GL-4 where GL-5 is specified unless the manufacturer explicitly allows it.
In practice, the majority of drivers will use a synthetic 75W-90 or 80W-90 GL-5 gear oil unless the manual specifies a different grade or a different viscosity for cold or extreme conditions.
Special considerations for limited-slip differentials
Limited-slip differentials (LSDs) add friction modifiers to oils to help transfer torque and prevent wheel spin. Using the wrong oil can reduce LSD effectiveness or cause chatter. Follow these guidelines if your car has LSD:
- Check whether your LSD requires an oil with built-in friction modifiers or if a separate additive is recommended by the manufacturer.
- Choose an oil labeled as LSD-compatible, or plan to add the approved friction-modifier additive if the oil does not include it.
- Verify compatibility with your specific LSD type (clutch-type, torque-sensing, or viscous) since some LSDs respond differently to certain modifiers.
When in doubt, consult your vehicle’s service manual or dealership service department for the exact LSD oil and modifier requirements for your model.
Maintenance tips: changing rear-differential oil
Regular service of the rear differential helps prevent wear and overheating. Here is a practical sequence to follow, with notes on what to watch for.
- Confirm the oil capacity and correct specification from the owner's manual or service bulletin before draining. Gather the correct fill and drain plugs, a catch pan, a wrench, and the appropriate oil grade.
- Warm up the vehicle slightly to get the oil flowing, then securely lift and support the vehicle on jack stands if needed.
- Place the catch pan beneath the differential, remove the fill plug (to gauge space) and then remove the drain plug to drain old oil completely. Allow several minutes for complete drainage.
- Clean and reinstall the drain plug with the correct torque specification. Refill slowly through the fill hole using a funnel until the fluid begins to trickle out of the fill hole, then stop. Reinstall the fill plug to the proper torque.
- If your vehicle has an LSD, add the manufacturer-recommended friction modifier or the LSD-specific oil as directed. Do not overfill.
- Wipe up any spills, lower the vehicle, and dispose of the used oil according to local regulations. Check for leaks after a short test drive and top off if necessary.
Following these steps helps ensure the rear differential remains properly lubricated and protected against heat and wear.
Summary
Choosing the right rear-differential oil comes down to matching the OEM specification (usually GL-5) and the correct viscosity (most often 75W-90 or 80W-90). If your vehicle has a limited-slip differential, verify LSD compatibility and whether a friction-modifier additive is required. For harsh climates, heavy towing, or high-load driving, higher-viscosity options like 85W-140 or 75W-140 may be appropriate. Always start with the owner’s manual or dealer guidance, and perform regular maintenance to protect gears, seals, and bearings over the long term.
