AUTOMOTIVE GEAR LUBRICANTS by Dave Mann, Truck/Automotive Engineer, Lubrication Specialist

This section will cover gear lubes in relation to automotive and some truck applications. There are numerous other types of gear lubes for industrial applications that utilize different types of gear sets, however they will not be covered in this book.  The SAE has established viscosity grades for gear lubes just as they have for engine oils.

The most common SAE viscosity grade gear lube used in production cars and light trucks is 80W-90.  75W-90 is also a very common viscosity grade gear lube. If your vehicle specifies 80W-90, as many production vehicles do, you can certainly use a 75W-90. There is very little difference between the two. According to SAE J306 specification the maximum temperature for a viscosity of 150,000 centipoise for a 75W is minus 40 deg. C. while the value for an 80W is minus 26 deg. C.  This simply means that the 75W is going to flow slightly better under colder temperatures than the 80W will, but nothing that you will ever be able to detect without very specialized test equipment.  Both viscosity gear lubes will protect equally well and are both 90 weight gear lubes at normal operating temperatures.

Many light, medium and heavy-duty trucks will specify an 75W-90 or an 85W-140. In the case of the 85W-140 you can use either an 85W-140 or a 75W-140. The differences are in cold weather flow properties and extremely minimal. The 75W-140 is going to flow slightly better than the 85W-140 in cold weather until the gear lube warms up in which case they will both perform as a 140 weight gear lube. Many synthetic gear lubes will show SAE grades of 75W because of their excellent low temperature characteristics. In the future 75W-110 gear lubes will become popular as manufactures strive to maximize protection while maximizing fuel economy.

Synthetic Gear Lubes

There are several manufacturers are using a synthetic 75W-140 gear lube in some light trucks and performance vehicles while a few also use a synthetic 75W-90 in some high performance cars and trucks.  One of the first production applications of a 75W-140 synthetic gear lube I can remember in a production light truck was in a very performance model that was having heat related problems in the rear axle that was causing the plastic housing of the 3-channel anti-lock system (ABS) sensor located in the differential housing to overheat and distort.

In a typical ABS system there is a toothed sensor at each wheel that measures wheel speeds and compares them using sophisticated computer technology and can determine which wheel or wheels is locked up and then send a signal to the ABS unit to modulate brake pressure.  This is called a 4-channel system.  A 3-channel ABS system is a less costly type of system and only has one sensor at the rear differential and one at each front wheel, thus the name 3-channel ABS system. The only way to solve the problem was to use a synthetic 75W-140 gear lube in the axle. The synthetic gear lube lowered the temperatures significantly and eliminated the problem, as well as provided for increased performance and durability of the gear set and axle bearings.  You will see that several manufacturers are now using and specifying synthetic gear lube for axles, especially in the rear axles of trucks. The synthetic gear lube is used for a reason. A manufacturer is generally not going to use a higher performance and more costly gear lube unless it is necessary to solve a specific problem or improve performance or durability issues.  The synthetic gear lubes that are used as a factory fill are superior to petroleum gear lubes.

When it comes time to change the factory fill synthetic gear lube I recommend purchasing one of the premium quality synthetic gear lubes manufactured by AMSOIL INC.  The performance is superior and the cost is significantly less that you will pay by purchasing it from the dealership. The specifications state that in non-commercial normal service car and light truck applications their gear lubes have a change interval specification of 100,000 miles. In severe service the change interval specification is 50,000 miles. For heavy trucks change intervals range up to 250,000 miles

If your vehicle originally came with a petroleum gear lube I recommend upgrading to a premium quality synthetic gear lube.  Synthetic gear lubes operate at different temperature ranges in relation to viscosity than petroleum gear lubes do.  This allows for one gear lube to cover a wide range of operational, temperature and load conditions from extreme cold to extreme heat.  PAO’s are the most commonly used base stocks for synthetic gear lubes for automotive applications and are formulated without any viscosity index improvers (VI’s) since PAO’s have a natural high viscosity index of the base fluid.  For example, “a synthetic 75W-90 gear lube will flow to temperatures as low as minus 40 deg. C. while a petroleum based 80W-90 gear lubricant’s flow is retarded at these very low temperatures (a)”.

Synthetic gear lubes are also is much more shear stable. A petroleum multi-viscosity gear lube will contain VI’s in order to meet the viscosity-spread requirements while the synthetic multi-viscosity gear lube will not. In a test of two different trucks using 75W-90 petroleum and synthetic gear lubes, the truck with the petroleum gear lube actually “sheared out of grade, lowering the viscosity considerably. The gears from this test showed excessive wear indicative of low viscosity.  By comparison, the truck with the synthetic based PAO showed very little shear loss and showed acceptable performance at the end of the test (a)”. Synthetic gear lubes are also superior in the areas of oxidative and thermal stability and there is significant data supporting this fact from both axle manufacturers and synthetic oil manufacturers, however it is not my intention to review the data in this book. Synthetic gear lubes will also allow for a reduction in lube temperature, easier shifting in automotive manual transmissions and gearboxes such as on motorcycles and also show a significant improvement in cold weather shifting performance

Friction Modifiers

It is also important to note that manufacturers also specify a friction modifier, typically 4-oz., to be added when gear lube is replaced in an axle that has a factory (or aftermarket) limited slip differential.  With some synthetic gear lubricants this is typically not the case. As discussed previously, synthetic lubricants are not more “slippery” than petroleum gear lubricants. I recently had a customer that was told he needed to use twice as much friction modifier with a synthetic gear lube due to the increased “slipperiness” of synthetics. Again, this is not true. I know of several premium quality synthetic gear lubricants that do not require any friction modifiers to be added with limited slip differentials. Check the gear lube manufacturers specification to determine if a friction modifier is required before putting it in. It will be of no benefit if you install it when it is not needed. The best way to determine if a friction modifier is required is to, after installing the synthetic gear lube in a limed slip axle, go out and drive the vehicle as you normally would but also be sure to make several tight right and left turns, or find a location where you can safely and slowly drive in a figure “8” pattern for a minimum of eight to ten repetitions.   If you notice any chatter on turns then you most likely need a friction modifier and you can take out the differential filler plug and add it very easily. If you do not get any chatter then you do not need friction modifier.

Extreme Pressure Additives and Performance

Another important aspect of gear lubes to consider is extreme pressure (EP) performance and additives. Both petroleum and synthetic gear lubricants utilize EP additives where film strength and extreme pressure performance are needed to combat high shock loading of the gears.  Sulfur, phosphorus and/or chlorine based additives are often used, although “chlorine is not fully effective or may cause corrosion concerns where water is present (d)”.  The most common EP additives are a sulfur-phosphorus combination in automotive gear lubes.

Extreme pressure additives are activated by heat rather than by pressure.  When activated by heat they then chemically react with the metal surfaces on the gears to form a film that wears or polishes off slowly rather than allowing the mating surfaces to weld together causing destruction of the gears, bearings and even axle shafts once metal particles and fragments are loose inside the axle housing and are circulated by the gear lube.

Note that it takes time in service for this protective film to develop but once that it has developed it has a lower shear strength than the metal used for the gears and thus reduces friction and heat. These sulfur and phosphorus compounds can also tend to be corrosive to non-ferrous parts. There are different GL ratings for different type of gear lubes and applications. Applications requiring a gear lube which is “friendly” to non-ferrous parts  (such as in marine outboard and stern drive lower unit gears and certain manual transmissions, which use aluminum or magnesium cases and other non-ferrous bushings and components) typically will specify a GL-4 rated gear lube which has a different EP additive chemistry and is compatible with the non-ferrous components.

The GL ratings are specified by the API and are as follows:

GL-1:  Specified for automotive spiral-bevel and worm gear axles and some manual transmissions subject to mild service and that can be sensitive to some EP additives.  These lubes usually contain rust and oxidation inhibitors, defoamants and pour point depressants but do not contain any EP additives or friction modifiers.

GL-2:  Specified for automotive worm gear axles that are subject to more severe service than that which is covered under the GL-1 specification.

GL-3:  Specified for manual transmissions and spiral-bevel axles operating under moderately severe service.

GL-4: Specified for hypoid gear service without shock loading, but still moderate to severe service (high speed/low torque and low speed/high torque).  These lubes may be used in manual transmissions and transaxles where EP additives are acceptable and typically contain a different zinc additive combination. This classification is still commonly used, but is also obsolete. It is also a commonly specified for marine outboard and stern drive lower unit gears and bearings where the manufacturer specifies GL-4 rated performance.  GL-4 rates lubes are not recommended as a replacement for GL-5 rated lubes. “GL-5 rated lubes shows significantly better anti-wear (anti-score) properties than GL-4 rated oils under similar service conditions (d)”

GL-5:  Specified for hypoid gears under shock loading and severe service operating conditions used in cars and trucks. This is the most common and widely used specification today.  These lubes have a high level of EP additives and, depending on the manufacturer and formulation, could be mildly corrosive to non-ferrous parts in certain applications.

GL-6:  This classification is obsolete and/or was never formally adopted by the API, although I have seen some racing type gear lubes advertised as “high-performance GL-6”.  It is more than likely advertised this way for marketing purposes.

API MT-1:  Specified for manual transmissions that do not have synchronizers. The oxidation level and thermal stability is higher than GL-1, GL-4 and GL-5 (z).

MIL-PRE-2150E:  Specified by the military for automotive gear lubes. It combines the requirements of API GL-5 and MT-1 (z).

Note that in although the previous lubricant specifications have generally provided satisfactory levels of performance for current automotive gear units, some difficulties have been experienced in low-temperature service because operating temperatures are not always high enough to fully activate the various EP agents.  This disadvantage is usually overcome by using a higher dosage level of additive in lubricants intended specifically for arctic-like service (z).  In cases such as arctic-like service synthetic gear lubricants will be a major improvement over petroleum based gear lubricants.  I know of some auto manufacturers that install synthetic lubricants in vehicles destined for areas that experience extended arctic weather conditions.

See the Complete Line of AMSOIL Synthetic Gear Lubricants