What kind of fluid goes in a rear differential?
Typically, the rear differential uses gear oil (not engine oil or automatic transmission fluid). Most setups call for GL-5 gear oil in a viscosity such as 75W-90 or 80W-90, with synthetic options common for better temperature stability; if the differential is limited-slip (LSD) or performance-oriented, an LSD-specific fluid or friction-modifier additive may be required as directed by the manufacturer.
What you need to know
The rear differential relies on gear oil to lubricate gears, bearings, and seals under a range of temperatures and loads. The exact specification—viscosity, whether synthetic is required, and whether an LSD additive is needed—depends on the vehicle and the differential type. Always consult the owner’s manual or the differential tag for the precise requirement.
Common rear-differential fluids
Here are the fluid options you are likely to encounter, along with what they mean for protection and performance.
- Conventional mineral gear oil: typically GL-5 rated, with viscosities such as 80W-90 or 85W-140. Suitable for many older or budget vehicles.
- Synthetic gear oil: often GL-5 or higher, with viscosities like 75W-90 or 75W-140. Offers better temperature stability, reduced wear, and longer change intervals, especially in extreme heat or cold.
- Limited-slip differential fluids (LSD): Some differentials require a fluid formulated for limited-slip units or an additive to achieve the proper friction characteristics. Manufacturer labels may indicate “LSD” or require a separate friction-modifier.
- High-performance or heavy-load fluids: Certain vehicles used for towing or high-performance driving may specify 75W-140 or a specialty synthetic with enhanced shear stability.
Concluding: Always verify the exact spec in your owner's manual or on the differential tag, and avoid using engine oil, ATF, or other non-gear-oil substitutes unless the manufacturer explicitly allows it.
Maintenance and selection tips
To choose and maintain the rear-differential fluid correctly, follow these guidelines:
- Check the vehicle’s manual or the differential’s fill cap/differential tag for the exact viscosity and the required type (conventional, synthetic, or LSD-rated).
- Determine whether your differential is standard or LSD; if LSD, ensure you have the correct friction modifier or LSD-specific oil as specified.
- Use the recommended oil, and avoid mixing different viscosities or brands unless the manual allows it.
- Replace at the factory intervals or earlier if you tow heavy loads, drive in extreme temperatures, or perform off-road work.
- When changing, ensure proper fill to the correct level per the service manual and replace any gaskets or crush washers as needed.
Concluding: Correct fluid choice and timely changes protect gears, bearings, and seals, reducing noise and wear, especially in demanding conditions.
Summary
Rear differentials typically use GL-5 gear oil in the appropriate viscosity, most commonly 75W-90 or 80W-90, with synthetic versions increasingly common. Some vehicles with limited-slip differentials require a friction-modifier additive or an LSD-specific oil. Always follow the manufacturer’s specifications, use the proper oil, and schedule regular changes to maintain protection and performance.
What type of fluid do I need for a rear differential?
Think engine oil but thicker; it's similar to fluid that's used to lubricate manual transmission gears. There are two basic types of differential fluid: Mineral oil, which is a natural, crude oil-based fluid. Synthetic differential fluid, which is created with chemicals and can be fine-tuned for optimal performance.
Which is better, 75W90 or 80w90?
Yes, 75W-90 is generally better than 80W-90 because it flows better in cold temperatures and offers better protection across a wider range of temperatures, leading to improved efficiency and longer drain intervals. The "75W" rating indicates better cold-weather performance compared to "80W," while both have a similar "90" rating for high-temperature viscosity. This makes 75W-90 a preferred choice, especially in climates with significant temperature swings or for applications prioritizing fuel efficiency and reduced maintenance.
Advantages of 75W-90 over 80W-90
- Better cold-start performance: The "75W" designation means the oil is thinner when cold, making it easier for the engine to start and providing better lubrication to internal components immediately.
- Wider operating temperature range: 75W-90 maintains a more consistent thickness across a wide temperature range, offering better protection in both hot and cold conditions.
- Improved fuel efficiency: The lower viscosity at colder temperatures can reduce drag and lead to a small improvement in fuel economy, as noted by OEMs.
- Longer service life: 75W-90, especially when synthetic, is more resistant to oxidation and thickening, allowing for longer drain intervals and reduced maintenance costs.
When 80W-90 might be sufficient
- Standard climate conditions: In regions with mild and stable climates, the difference in performance may be minimal, and 80W-90 could be adequate.
- Budget-friendly choice: 80W-90 is often less expensive upfront than 75W-90. However, the total cost of ownership can be higher when factoring in reduced efficiency and shorter drain intervals.
Summary
| Feature | 75W-90 | 80W-90 |
|---|---|---|
| Cold Flow | Superior (flows better in winter) | Adequate (flows less easily when cold) |
| High-Temperature Viscosity | Same (90 weight) | Same (90 weight) |
| Performance | Better for extreme climates and efficiency | Adequate for standard climates |
| Cost | Higher initial cost | Lower initial cost |
| Maintenance | Longer drain intervals | Shorter drain intervals |
(this||self).Wufxzb=function(c,e,f,l,k){var d=document.getElementById(c);if(d&&(d.offsetWidth!==0||d.offsetHeight!==0)){c=d.querySelector("div");var g=c.scrollWidth-c.offsetWidth,h=Math.min(e?g:0,g);c.scrollLeft=e&&(l||f)?0:h;var a=d.getElementsByTagName("g-left-button")[0],b=d.getElementsByTagName("g-right-button")[0];a&&b&&(e=RegExp("\\btHT0l\\b"),f=RegExp("\\bpQXcHc\\b"),a.className=a.className.replace(e,""),b.className=b.className.replace(e,""),h===0?a.className="pQXcHc "+a.className:(a.className=
a.className.replace(f,""),k&&c.classList.add("pA30Ne")),h===g?b.className="pQXcHc "+b.className:(b.className=b.className.replace(f,""),k&&c.classList.add("FpCCub")),setTimeout(function(){a.className+=" tHT0l";b.className+=" tHT0l"},50))}};}).call(this);(function(){var id='_990lacXyM9Ti7_UPmqqimAQ_369';var rtl=false;var gecko=false;var edge=false;var soh=false;
(this||self).Wufxzb(id,rtl,gecko,edge,soh);})();
In most modern vehicles and climates, especially where efficiency and protection are priorities, 75W-90 is the better, more modern choice. However, always consult your vehicle's owner's manual to ensure you are using the manufacturer's recommended oil viscosity.
Can you put transmission fluid in a rear differential?
While they both play crucial roles in the overall performance of your vehicle, it's essential to understand that differential fluid and transmission fluid are not interchangeable.
Does it matter what diff oil I use?
You will never feel any difference with new or old diff-oils, even with minor different viscosity or GL-spec. Differences, if any, will only present themself in the long run (durability related).
