What is Differential Fluid | Gear Oil Weight Explained
What is Differential Fluid?
Much like how engine oil provides a protective layer on moving engine components, differential fluid, often simply referred to as gear oil, does much the same thing for your vehicle’s differential components. While your engine uses an oil pump to provide pressurized oil to the engine’s bearings, lifters, etc. the differential uses “splash” oiling by way of the movement of the ring and pinion gears slinging the oil throughout the differential housing. This differential fluid is used to lubricate the gears, differential and axle bearings, and other components within the axle housing. This gear oil coating also provides heat dissipation of the heat generated by the friction of the ring and pinion gear contact (in racing environments some vehicles use a cooler and pump to circulate the gear oil to an external cooler and then back to the housing). The gear oil also acts as a corrosion inhibitor, acting as a protective oil film coating on steel and cast iron components inside the differential and axle housing.
Gear Oil Weight Explained
What is gear oil weight? A gear oil’s weight, or viscosity, is measured like engine oil by its thickness with a numerical value associated with it—albeit much thicker. Where engine oil may be in the 10W30 range for a multi-weight oil, gear oil is more often in the 75W90 range. Like engine oils, the first number shows the oil’s thickness when cold, while the second number denotes the thickness value once the oil is at operating temperature. Viscosity is the oil’s resistance to flow. Using the correct viscosity oil (and any differential oil additives) is critical for proper operation of your differentials internal components, as well as the ring and pinion gears themselves. Higher viscosities suit extreme usage needs but can be too thick for some differentials to work properly. It is always best to use what the differential manufacturer recommends in most cases for optimum torque transfer and axle oil temperature, while usage and climate will sometimes provide slightly different viscosity needs. Lastly, just like specialty engine oils, you can find gear oil in single weight viscosity offerings as well.
Mixing different weights or brands of gear oil is generally a bad practice, as it can compromise the performance and protection of the gear assembly (too low a viscosity, not enough friction modifier, etc.). Mixing additive packages, viscosity ratings, and even base oils are all conditions that may lead to reduced effectiveness of the gear oil’s capabilities. Topping off your differential with an unknown oil (or not knowing what gear oil is in the differential) is not recommended. So instead of asking “Can I use 80W-90 instead of 75W-90?” to top off your differential, it is best to drain the differential and refill with the correct amount and type of fluid with the proper additive package your differential requires.
Gear Oil Weight Chart
What Weight Gear Oil for Rear Differential?
As noted above, it is often best to start with the axle/differential manufacturer’s suggested gear oil weights, and then, if necessary, adjust the viscosity for your climate and use conditions. We’ve provided a list here of the most popular rear axle housings/differentials and their recommended factory gear oil spec for you to start with. Always refer to the manufacturer's recommendations for specific differential oil additives or limited-slip differential requirements and correct rear differential oil capacity.
- Gear Oil for GM 10 Bolt: 75W-90 or 80W-90
- Gear Oil for GM 12 Bolt: 75W-90 or 80W-90
- Gear Oil for Ford 8-Inch: 75W-90 or 80W-90
- Gear Oil for Ford 9-Inch: 75W-90 or 80W-90
- Gear Oil for Dana 30: 75W-90 or 80W-90
- Gear Oil for Dana 60: 75W-90 or 80W-90
Limited Slip Differential Fluid VS Posi Rear End Fluid
When it comes to a limited slip differential (LSD) and a Posi differential, one must understand that the name “Posi” is a GM term for a limited slip diff. So, the fact is all Posi differentials are limited-slip in nature, but not all limited-slip diffs are in fact a “Posi” unit. For example, Ford has used the terms Equa-Lok and Traction-Lok over the years for their LSD, while Mopar has used the Sure Grip name. Many of these limited slip units use some sort of friction material like what you’d find in an automatic transmission clutch pack. These clutch discs or cones reduce wheel spin and direct the driveline’s output to the wheel with higher traction.
Limited slip/Posi units typically use a specific type of gear oil or differential fluid. The exact gear oil required can vary depending on the manufacturer's specifications and the type of limited-slip differential installed in the vehicle. Some limited-slip differentials may require a special additive, such as a friction modifier, in the fluid to ensure proper operation and clutch friction. This can be a separate bottle of differential oil additives or a gear oil with the additive properties built into it. Some aftermarket limited-slip units do not have friction materials at all, but springs or even ratcheting teeth, and do not require this additive, which is why it is imperative you follow the differential manufacturer’s recommendations for fluid type and viscosity.
What Gear Oil to Use for Drag Racing?
Often you will find in a drag racing application that the rear axle is not using a traditional clutch or cone-type friction-style differential, but something like either a locker or a spool. A locking differential may have friction material plates that require friction modifier additives (once again, refer to the manufacturer’s instructions for details/requirements). There are electro-magnetic lockers, mechanical cable and rod actuated lockers (usually only seen in off-road use), and traditional gear lockers that disengage during cornering, providing that classic “ratcheting” rear axle noise. Spools, on the other hand, are literally a machined piece that the ring gear is bolted to and provides direct connection to both axles. With a spool installed you’ll often hear the tell-tale tire chirp when cornering. Both are not street friendly and can affect handling and tire wear adversely, but that doesn’t mean people still don’t use them on the street!
When it comes to gear oil for drag racing with a traditional differential, a locker, or even a spool, the load on the ring and pinion gears due to the much higher horsepower and torque output and the prepared track surface combined with a slick or drag radial’s sticky compound is much higher. As such, you want to use a “severe” service type of gear oil and often something with higher shear properties that come with a thicker oil, such as a 75W110 or 75W140. We even offer up to a 75W190 if needed.
How to Check Rear Differential Fluid Level
Obviously, having the proper amount of gear lube in your rear axle housing to keep your differential running cool is critical. The difficult part is that the differential is under the rear of the car with no straightforward way to check its fluid level. Sorry, you can’t simply open the trunk and grab the differential’s dipstick for a quick check like you can your engine or automatic transmission. No, you’re going to have to put in a little work and get under the car. We will suggest though, with a little bit of work, you can install a rear end fluid level checker in your differential’s casting (be sure you’re not drilling into a bearing or other critical area) or differential cover if it is an integral carrier style rear axle (Ford 8.8, GM 10-bolt, etc.) to make these checks a bit easier.
Traditional differential fluid checks require the removal of the oil fill plug (usually located on the center section but may be in the differential housing cover). To access this means safely jacking up the car in most cases and placing it on jack stands. We suggest doing the same for the front of the car to maintain a level driveline for checking the gear oil. Remove the fill plug after cleaning the area of any dirt and use a small tool or your finger as a “gauge” to confirm fluid level. A properly filled differential will have the oil level at the bottom edge of the fill plug opening. Your tool or finger should come out wet with gear oil. If it doesn’t you will need to top off your differential until the gear oil just starts to run out of the fill plug opening. Reinstall the fill plug and you’re good to go.
When to Change Differential Fluid?
It is easy to forget about the rear axle oil inside the differential under your car. It is the epitome of “out of sight, out of mind.” However, just like your engine needs oil changes at regular intervals, so does your differential. The frequency of these oil changes will depend on the use and driving conditions, but as a general guideline we recommend draining and filling your differential with fresh gear oil every 30,000 miles. Obviously, if you’re “in there” servicing the differential or axle housing, say installing new axle bearings, it would be wise to go ahead and grab some fresh gear oil for when you go back together.
Integral carrier axles tend to be a bit easier, as you simply remove the differential cover to drain the old lube. However, some differential carriers have both drain and fill plugs, or you can drill and tap for a drain plug as well. Lastly, we offer numerous differential fluid change kits that include a fluid pump that you can use to pull the old fluid out and pump the new fluid in without having to remove the differential carrier or cover. This makes fluid changes quick once you have the vehicle safely on jack stands for access.
Which Gear Oil is the Best?
It can be hard to provide an absolute answer as to what is “best” for most anything, as what is best for one application may not be the best for another. Just about every popular rear axle still in use today is a hypoid gear design, meaning it is a spiral bevel gear design where its axis does not intersect with the axis of the mating gear. These gears require anti-wear and pressure additives to protect the gears from the high loads imparted on them. While you may be asking “What differential fluid do I need?” the better questions may be “Why change differential fluid?” and “When change differential fluid?” We’ve answered these “why and when” questions under other headings of this article.
Most gear oils today will have the proper additives, but the best way to be certain is to confirm the American Petroleum Institute (API) rating of the oil. API GL-4 is the classification for oils used mainly in transmissions or other applications with helical or spur gear configurations and does not provide the proper protection for hypoid gear applications. API GL-5, however, is rated to handle the high pressure contact of hypoid gear applications with the use of a specific additive package. You may find some dual rated oils, but for differential use, always look for the API GL-5 rating on the packaging for the proper hypoid gear oil.
One of the most asked questions pertaining to gear oil is about using conventional or synthetic gear oil. Conventional oil is made up of crude oil and petroleum base materials and any necessary additives. Synthetic oil is made from chemically modified compounds made in a laboratory. Synthetic oils have a more uniform molecular structure for enhanced performance and stability, which helps under higher loads and temperatures where a conventional base oil may breakdown. This provides additional protection in extreme applications/use where a conventional base oil would no longer be providing protection.
Furthermore, synthetic oils are more viscosity stable, meaning that they maintain their viscosity/weight through a wider temperature range. While more expensive than a base oil product, synthetic oils often provided extended drain intervals and higher protection. So, at the end of the day the “best” oil may be a basic 75W90 conventional gear oil for your race car that sees a lot of differential/gear swaps, while your daily driven pickup might be better served with a synthetic version for the added heat and pressure the pickup sees towing the race car (and the extended drain interval meaning you don’t have to change it as often).
Differential Oil Additives
Gear oil additives can be a separate product or part of the gear oil’s base oil additive package for an “all in one” gear oil solution. These friction modifiers are critical to the proper operation of clutch type limited slip or Posi differentials. The clutch friction material found in such a unit can chatter or cause driveline vibrations. Adding friction modifier additives smooths out this clutch engagement, reduces chatter and overall driveline noise. Smoother engagement means less wear on the clutch friction materials and quicker lock up of the clutch pack or cones for enhanced traction and stability.
Most gear oils do not include friction modifier or other additives required of a clutch-style limited slip differential, requiring the addition of a bottle of friction modifier to mix in with the standard gear oil. More gear oil manufacturers, however, are beginning to offer specialized limited slip differential fluid explicitly formulated for these units. These limited slip gear oils provide the proper additive package for clutch-type Posi units right in the bottle with nothing else to add, making gear oil changes or top-offs convenient and trouble free.
