What You Need To Know About Changing Rear Axle Lubricant

Wayne Scraba

1 Dec 2023

Click Here to Begin Slideshow The axle lubricant in your rear-wheel drive car or truck lives in an extreme environment. Temperatures can exceed 300-degrees F under certain circumstances. And aside from a select bunch of super high-performance automobiles, few vehicles are manufactured with provisions to cool the fluid. Under winter conditions, the rear axle has no provision for warming. You simply warm the engine, then drive away. That means the fluid can be extremely cold (and likely very thick – fluids have large viscosity numbers – many in the 80W to 90W range). There’s more: Common production line rear axle assemblies don’t have pumps and they don’t have filters (although they can be found in several different forms of motorsports). That means the gear lube is distributed by way of splash action (don’t forget the ring gear is constantly churning through the lubricant), and not one ounce of the fluid is filtered. Worse, the rear axle isn’t immune to contamination. Most have a vent somewhere on the housing (typically on the axle tube). Unfortunately, the vent is in a vulnerable location. It’s susceptible to road debris, water, road grime and so on. In a vehicle that sees off-road use, you can well imagine how much dirt and foreign fluid can actually enter the rear axle assembly by way of the vent. So what can you about it? The answer is simple: Change the gear lubricant on a regular basis. And yes, we know it tends to be a messy and equally stinky job. Many manufacturers advocate rear axle lubricant changes every two years or every 25,000 miles, whichever comes first. What do you require for a lubricant? It depends upon the vehicle. For example, the 2008 K1500 Chevy pickup shown in the photos mandates SAE 75W-90 Synthetic Axle Lubricant. Earlier vehicles may mandate something far different. The Mark Williams built 12-bolt in the writer’s street machine mandates a minimum SAE 80W or SAE 80W-90. It has limited slip assembly and as a result, mandates a lubricant with a friction modifier. That means it requires special limited slip rear axle lubricant. More in the accompanying photos. Today, there are several different lubricants available engineered specifically for today’s automobiles as well as older applications. What you need for a rear axle assembly (old or new) is a thermally stable, extreme-pressure gear lubricant. That lubricant must prove stable under considerable temperature swings, and simultaneously, it must be capable of withstanding considerable shock loading (basically, that’s what kills rear axle gears). There’s more too: The rear axle lubricant must have excellent anti-wear properties; it must provide anti-scuff protection and it must also provide protection against corrosion. Keep in mind the fluid also must have anti-foaming agents. Since the (big) ring gear is constantly churning in a pool of lubricant, it can aerate the lube. Foam doesn’t lubricate well. Finally, the lubricant must be capable of working with different types of limited slip (Detroit Locker, Positraction, etc.) assemblies. If the rear axle lubricant does not this capability, then you’ll need some sort of friction modifier additive package. Commercially available friction modifiers are formulated for use in limited-slip, Positraction and locking differentials to reduce hypoid gear noise and chatter. The various formulations modify clutch plate friction for applying and releasing clutch plates. If you need to use one of these friction modifiers, keep in mind that GM limited slip (Positraction) rear axle assemblies differ from those used in Ford, Chrysler and Toyota applications, although some of the friction modifiers out there are universal. So far so good, but what should you expect if you change the lube or have someone perform the task for you? Changing rear axle lube is often a messy task, particularly for cars and trucks that don’t have a drain plug. If there is no drain plug, you’ll need a fresh rear cover gasket (or in the case of a front-loading rear axle like a Ford 9-inch, a center section gasket). You’ll also need a good-sized drain pan (one that can hold roughly three to four quarts of fluid). In order to button everything up, you’ll also need a bit of RTV sealer or other gasket sealer, the proper lubricant and if required, limited slip friction modifier. Due to the viscosity of the lubricant, the rear axle should be warmed up (driven for a few miles) before the drain and refill process begins. At this point, the rear axle inspection cover is removed, and the fluid is drained. This is accomplished by loosening the respective inspection cover bolts and gently prying the lowest corner of the cover loose. Once the majority of the fluid drains, the cover can be completely removed. The gasket mounting surfaces should be cleaned of all old gasket material and excess lubricant (on the gasket mounting surfaces) should be removed. Brake cleaner does a good job of removing oil residue. At this point, a new cover gasket can be installed (using gasket sealer or RTV), and the cover can be reattached to the rear axle. Cover bolts should be torqued to spec. Next, the rear axle is filled – first with the friction modifier package, then with the rear axle lubricant (filler ports for most housings are close to the center section, often over one of the axles) to the vehicle manufacturer’s recommended level. When adding friction modifiers, shake the small bottle vigorously before pouring into the rear axle assembly. Often I warm the lube in a microwave oven for a short amount of time before installation – it definitely pours easier. Each vehicle manufacturer has their own respective fluid level specifications (for each rear axle assembly). Check the service or owner’s manual for specifications. If your vehicle has a drain plug on the rear axle, you can skip the rear inspection cover removal and replacement process. In this case, it's a simple job that proves similar to an engine oil change, less the filter part of the equation: Completely drain the old fluid, reinstall the drain plug and refill the axle assembly. Once the fluid is changed, it’s a good idea to very carefully start the vehicle and operate the rear axle (properly chock the front wheels, place in drive or in gear and allow to operate at engine idle speeds) while it is on axle stands. This allows the rear axle to function. Alternatively, you can take the vehicle for a short drive around the block. Inspect for leaks. The job is done. Click Here to Begin Slideshow

What You Need To Know About Changing Rear Axle Lubricant — Click Here to Begin Slideshow The axle lubricant in your rear-wheel drive car or truck lives in an extreme environment. Temperatures can exceed 300-degrees F under certain circumstances. And aside from a select bunch of super high-performance automobiles, few vehicles are manufactured with provisions to cool the fluid. Under winter conditions, the rear axle has no provision for warming. You simply warm the engine, then drive away. That means the fluid can be extremely cold (and likely very thick – fluids have large viscosity numbers – many in the 80W to 90W range). There’s more: Common production line rear axle assemblies don’t have pumps and they don’t have filters (although they can be found in several different forms of motorsports). That means the gear lube is distributed by way of splash action (don’t forget the ring gear is constantly churning through the lubricant), and not one ounce of the fluid is filtered. Worse, the rear axle isn’t immune to contamination. Most have a vent somewhere on the housing (typically on the axle tube). Unfortunately, the vent is in a vulnerable location. It’s susceptible to road debris, water, road grime and so on. In a vehicle that sees off-road use, you can well imagine how much dirt and foreign fluid can actually enter the rear axle assembly by way of the vent. So what can you about it? The answer is simple: Change the gear lubricant on a regular basis. And yes, we know it tends to be a messy and equally stinky job. Many manufacturers advocate rear axle lubricant changes every two years or every 25,000 miles, whichever comes first. What do you require for a lubricant? It depends upon the vehicle. For example, the 2008 K1500 Chevy pickup shown in the photos mandates SAE 75W-90 Synthetic Axle Lubricant. Earlier vehicles may mandate something far different. The Mark Williams built 12-bolt in the writer’s street machine mandates a minimum SAE 80W or SAE 80W-90. It has limited slip assembly and as a result, mandates a lubricant with a friction modifier. That means it requires special limited slip rear axle lubricant. More in the accompanying photos. Today, there are several different lubricants available engineered specifically for today’s automobiles as well as older applications. What you need for a rear axle assembly (old or new) is a thermally stable, extreme-pressure gear lubricant. That lubricant must prove stable under considerable temperature swings, and simultaneously, it must be capable of withstanding considerable shock loading (basically, that’s what kills rear axle gears). There’s more too: The rear axle lubricant must have excellent anti-wear properties; it must provide anti-scuff protection and it must also provide protection against corrosion. Keep in mind the fluid also must have anti-foaming agents. Since the (big) ring gear is constantly churning in a pool of lubricant, it can aerate the lube. Foam doesn’t lubricate well. Finally, the lubricant must be capable of working with different types of limited slip (Detroit Locker, Positraction, etc.) assemblies. If the rear axle lubricant does not this capability, then you’ll need some sort of friction modifier additive package. Commercially available friction modifiers are formulated for use in limited-slip, Positraction and locking differentials to reduce hypoid gear noise and chatter. The various formulations modify clutch plate friction for applying and releasing clutch plates. If you need to use one of these friction modifiers, keep in mind that GM limited slip (Positraction) rear axle assemblies differ from those used in Ford, Chrysler and Toyota applications, although some of the friction modifiers out there are universal. So far so good, but what should you expect if you change the lube or have someone perform the task for you? Changing rear axle lube is often a messy task, particularly for cars and trucks that don’t have a drain plug. If there is no drain plug, you’ll need a fresh rear cover gasket (or in the case of a front-loading rear axle like a Ford 9-inch, a center section gasket). You’ll also need a good-sized drain pan (one that can hold roughly three to four quarts of fluid). In order to button everything up, you’ll also need a bit of RTV sealer or other gasket sealer, the proper lubricant and if required, limited slip friction modifier. Due to the viscosity of the lubricant, the rear axle should be warmed up (driven for a few miles) before the drain and refill process begins. At this point, the rear axle inspection cover is removed, and the fluid is drained. This is accomplished by loosening the respective inspection cover bolts and gently prying the lowest corner of the cover loose. Once the majority of the fluid drains, the cover can be completely removed. The gasket mounting surfaces should be cleaned of all old gasket material and excess lubricant (on the gasket mounting surfaces) should be removed. Brake cleaner does a good job of removing oil residue. At this point, a new cover gasket can be installed (using gasket sealer or RTV), and the cover can be reattached to the rear axle. Cover bolts should be torqued to spec. Next, the rear axle is filled – first with the friction modifier package, then with the rear axle lubricant (filler ports for most housings are close to the center section, often over one of the axles) to the vehicle manufacturer’s recommended level. When adding friction modifiers, shake the small bottle vigorously before pouring into the rear axle assembly. Often I warm the lube in a microwave oven for a short amount of time before installation – it definitely pours easier. Each vehicle manufacturer has their own respective fluid level specifications (for each rear axle assembly). Check the service or owner’s manual for specifications. If your vehicle has a drain plug on the rear axle, you can skip the rear inspection cover removal and replacement process. In this case, it's a simple job that proves similar to an engine oil change, less the filter part of the equation: Completely drain the old fluid, reinstall the drain plug and refill the axle assembly. Once the fluid is changed, it’s a good idea to very carefully start the vehicle and operate the rear axle (properly chock the front wheels, place in drive or in gear and allow to operate at engine idle speeds) while it is on axle stands. This allows the rear axle to function. Alternatively, you can take the vehicle for a short drive around the block. Inspect for leaks. The job is done. Click Here to Begin Slideshow

There are all sorts of lubricants out there. My choice is Torco GL-6 Racing Gear oil has proven to be ideal for drag racing and oval track applications. SAE 85W-140 can be used when running a spool, posi-traction, or open differentials. It provides superior adhesive strength and extreme anti-score protection. In addition, the Torco gear oil available from MW is specially blended to include the friction modifying additives required for most clutch type Positraction units. Works great for me.

This particular custom fabricated rear axle assembly is equipped with both an inspection port (blue cap) and a drain plug. That means there is no need to remove the center section (it’s a front-loading Ford style 9-inch) to replace the fluid. If the center section mandates removal, then the axles have to be removed first. That usually means the brake drums or rotors have to come off too. It’s no small task.

In contrast, this Chevrolet Silverado has a rear axle assembly complete with a cover. In “rear loading” rear axle applications with no drain plug, the back cover must be removed in order to replace the fluid.

For many cars, there are no hard and fast rules for rear axle lube changes. Some have the service intervals spelled out in the owner’s manual or factory shop manual. Others don’t. As a rule of thumb, it’s a good idea to change the rear axle lubricant every 25,000 miles. And if the vehicle has a rough service life (for example, a race car, a truck used off-road or a street-strip car), then the fluid should be changed more frequently.

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