Viscosity Grade (SAE J306) A lubricant’s primary function is to reduce friction and wear, and its most important property is its viscosity (thickness/resistance to flow). Lubricants are considered incompressible and under ideal conditions maintain a constant layer of protection, known as film strength, to keep moving parts from contacting each other. With no direct contact, wear is eliminated. There is a point, however, at which heavy loads exceed the oil’s ability to separate parts and metal-to-metal contact occurs. This is, in part, a function of viscosity. The higher the viscosity of a lubricant, the greater the load it can carry. Using gear lube that is too thick, however, has disadvantages. Thicker oils are more difficult to circulate, particularly in cold temperatures, and wear protection can be sacrificed. Thicker gear lubricants also require more energy to circulate, which negatively impacts fuel economy. Additionally, thicker gear lubes have higher internal resistance (intra-fluid friction) which causes them to run hotter. There is no advantage to using a gear lube with a viscosity greater than that required by the application. Conversely, gear lube that is too thin will not have sufficient load-carrying ability to meet the equipment requirements. The SAE has developed a grading system, SAE J306, which categorizes gear lubricants based on their high- and low-tem- perature viscosities. An additional requirement of SAE J306 is shear stability, which is explained later in this document. The viscosity requirements for SAE 75W-90 gear lubricants are Max. Temperature for Viscosity of Kinematic Viscosity at 100°C (cSt)3 150,000 cP (°C)1,2 at (-40) expected Kinematic Viscosity at 100°C (cSt)3 is 41

Viscosity Index (ASTM D-2270)

 CLICK to look at this Gear Oil chart comparison using the ASTM D-2270 standard comparing AMSOIL Severe Gear 75W-90, Castrol SYNTEC 75W-90, GM Synthetic Axle 75W-90, Lucas 75/90 Synthetic, Mobil 1 Synthetic 75W-90, Mopar Synthetic 75W-90 with Mopar LS additive, Pennzoil Synthetic 75W-90, Red Line Synthetic 75W-90,Royal Purple Max-Gear 75W-90, Torco SGO Synthetic 75W-90 with Torco Type G LS additive, Valvoline SynPower 75W-90,Castrol Hypoy C 80W-90, Pennzoil Gearplus 80W-90, Valvoline High Performance 80W-90

Oil viscosity is affected by temperature changes during use. As a gear lubricant’s temperature increases, its viscosity decreases, along with load-carrying ability. The degree of change that occurs is determined by ASTM D-2270 and referred to as the lubricant’s viscosity index (VI). ASTM D-2270 examines the viscosity change that occurs between 40°C (104°F) and 100°C (212°F) . The higher the VI, the less the viscosity changes with temperature. A high VI is desirable and, in part, indicates high- er lubricant quality. It does not, however, represent a lubricant’s high-temperature viscosity or its load-carrying ability.
Similar to 5W-30 automotive engine oils, 75W-90 gear lubricants are defined as multi-viscosity. This means the gear lubricant has enough viscosity to protect against wear at high temperatures, as well as good flow properties at cold temperatures. Many gear lubes cannot fulfill both requirements without the use of VI improver additives. VI additives keep lubricants from becom- ing too thick to flow in cold temperatures and too thin to protect in high temperatures. VI additives have many uses. If used improperly in gear lubricants, however, they can break down and lose viscosity through a process called shearing. Because of this, the SAE incorporated the CEC L-45-A-99 (KRL) 20-Hour Shear Test as a requirement for all automotive gear lubes. This specification requires that gear lubes not shear down and fall below the minimum viscosity for that grade.

The KRL Test note:Lucas 75/90 Synthetic, at 22.35 cSt, and Royal Purple Max-Gear 75W-90 both fail the KRL test

click  to read this Gear Oil chart comparison chart using the 20 Hour KRL standard comparing AMSOIL Severe Gear 75W-90, Castrol SYNTEC 75W-90, GM Synthetic Axle 75W-90, Lucas 75/90 Synthetic, Mobil 1 Synthetic 75W-90, Mopar Synthetic 75W-90 with Mopar LS additive, Pennzoil Synthetic 75W-90, Red Line Synthetic 75W-90,Royal Purple Max-Gear 75W-90, Torco SGO Synthetic 75W-90 with Torco Type G LS additive, Valvoline SynPower 75W-90,Castrol Hypoy C 80W-90, Pennzoil Gearplus 80W-90, Valvoline High Performance 80W-90

The KRL Test utilizes a tapered roller bearing and test cup filled with 40 ml. of gear lube. The test parameters are set at 60°C (140°F), 1475 rpm, 5000 N load for a duration of 1,740,000 motor revolutions (approximately 20 hours). Each gear lube’s viscosity was recorded before and after the shear test as seen in the following graph.
This graph shows the initial oil viscosity and the viscosity after the shear test. The SAE J306 high-temperature viscosity requirements (shaded area) for SAE 90 gear lubes are between 13.5 centistokes (a unit of measure for viscosity designat- ed as cSt) and 18.49 cSt @ 100°C (212°F) maximum.
The test shows a graph of Lucas 75/90 Synthetic, at 22.35 cSt, and Royal Purple Max-Gear 75W-90, at 19.32 cSt, both exceed the maximum 18.49 cSt initial viscosity (red), failing the SAE J306 requirements for SAE 90 gear lubricants. All other gear lubricants were within the required high-temperature viscosity range prior to the KRL Shear Stability Test.
Viscosity measurements following the KRL Shear Stability Test revealed that seven gear lubes sheared down below the minimum viscosity requirements (orange), failing the shear stability requirements of the SAE J306. The two gear lubes with the largest viscosity loss, as reflected in the following graph, were Royal Purple, losing 40.6% of its viscosity, and Torco SGO Synthetic, losing 35.2% of its viscosity. Royal Purple was the only gear lube to fail both the initial viscosity require- ments and the shear stability requirements. It started out too thick and ended up too thin. Torco SGO Synthetic, which had the highest VI in the previous graph, finished the shear stability test as the thinnest of all the oils at 9.97 cSt, far below the minimum 13.5 cSt requirement. Lucas 75/90 Synthetic, with an initial viscosity that exceeded the maximum requirements by 20.8%, passed the shear stability test, but lost 34.5% of its viscosity, the third largest loss of viscosity. Both OEM gear lubes, GM and Mopar, failed the minimum viscosity requirements after the shear test. Of all the gear lubes tested, half did not meet the SAE J306 shear stability requirements.
AMSOIL Severe Gear 75W-90 was in the proper initial viscosity range and retained the highest viscosity after the shear test with a viscosity of 16.03 cSt – the mid-point of the SAE 90 viscosity grade.

Low-Temperature Viscosity - Brookfield Viscosity Test (ASTM D-2983)

As temperature decreases, the viscosity of oil increases. Gear lubricants with high viscosity at cold temperatures are less efficient, and the gears require more energy to turn. Gears and bearings in the differential and axle housing are splash- lubricated, and gear lubricants that are too thick at cold temperatures can starve internal components of lubrication, which can cause failure.
The cold-temperature viscosity of gear lubricants is indicated by the first number in the SAE viscosity grade (75W of a 75W- 90 gear lube). The SAE J306 standard utilizes the Brookfield Viscosity Test, recorded in centipoises (cP), to determine cold- temperature performance. The maximum viscosity is 150,000 cP at the given temperature for the SAE viscosity grade. For example, SAE 75W must be less than 150,000 cP at -40°C (-40°F), while SAE 80W must be less than 150,000 cP at-26°C (-15°F).
In the Brookfield Viscosity Test, a glass test tube is filled with gear lube and cooled to the appropriate temperature. A small spindle is inserted into the lubricant and the maximum torque required to rotate the spindle is recorded. The torque read- ing is used to calculate the viscosity in cP.
Cold-temperature performance is impacted by a lubricant’s high-temperature viscosity. High-viscosity gear lubes tend to have worse cold-temperature performance than low-viscosity gear lubes. AMSOIL Severe Gear, however, with the highest after-shear viscosity, exhibited the best cold-temperature properties of all gear lubes, except for Torco SGO, which thinned out of grade in the shear test. Royal Purple and Lucas failed the cold-temperature Brookfield requirements for 75W gear lubes, as well as the high-temperature requirements for SAE 90 gear lubes, effectively disqualifying them entirely from the SAE 75W-90 category. Royal Purple Max-Gear, having also failed the Shear Stability Test, was the only gear lube to fail every parameter of the SAE J306 requirements. Red Line was 14,100 cP over the maximum allowable viscosity at 164,100 cP, and Castrol SYNTEC 75W-90 had a borderline pass at 149,850 cP. As noted, SAE 80W-90 gear lubes are measured at -26°C (-15°F) and all test candidates passed.
*Red Line, Royal Purple and Lucas, having failed the viscosity requirement for SAE 75W, were then tested at the SAE 80W parameters for comparison purposes. Red Line scored 18,250 cP and Royal Purple scored 24,700 cP, showing better per- formance than the SAE 80W-90 gear lubes. Lucas, however, at 98,050 cP, showed worse cold-temperature properties than Castrol 80W-90, which is reflected in the overall score on page 19.

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