SAE VISCOSITY GRADE
Gear lubricants must flow freely when the axle is cold, yet have sufficient thickness or viscosity to separate the lubricated parts parts at normal operating temperatures.
The SAE classification for axle and transmission lubricant viscosity is indicated in the table below. Each viscosity grade has distinct criteria for low and high temperature performance.
|AXLE AND MANUAL TRANSMISSION LUBRICANT VISCOSITY CLASSIFICATION SAE J306 (February 2019)|
|SAE Viscosity Grade||Max. Temperature for Absolute Viscosity of 150,000 cP (°C)(1)||Kin. Viscosity @ 100°C, cSt(2)|
NOTE – 1cP = 1 mPa.s; 1 cSt = 1mm2/s
1. Using ASTM D2983.
2. Using ASTM D445.
3. Limit must also be met after testing in CEC L-45-A-99, Method C (20 hours).
SAE viscosity selection should be based on the minimum and maximum service temperatures. Today’s most commonly used gear lubricants are multi-grades (e.g. 75W-90, 80W-90 and 85W-140). These fluids meet both the low and high temperature requirements for the combined grades. For example, an 80W-90 oil must have the low temperature fluidity of an 80W as well as the viscosity of a 90 grade at higher temperatures.
API SERVICE DESIGNATIONS
Automotive gear lubricant performance is defined by the type of service it can be expected to perform satisfactorily. The API service designations were developed for manufacturers and end-users to assist them in selecting manual transmission or gear lubricants for a variety of operating conditions.
The API service designations range from GL-1 to GL-5 and describe gear lubricants in terms of general type, severity of service and application. The following table lists these designations.
The most commonly specified and available type of automotive gear lubricant in North America is API GL-5 and for manual transmission applications API MT-1. In Europe and other parts of the world where manual transmissions are more prevalent, API GL-4/MT-1 oils are used as frequently as API GL-5.
|API SERVICE CLASSIFICATION|
|GL-1 (inactive)||Straight mineral oil (inactive)||Automotive manual transmissions|
|GL-2 (inactive)||Usually contains fatty materials (inactive)||Worm gear drives, industrial gear oils|
|GL-3 (inactive)||Contains mild EP additive (inactive)||Manual transmissions and spiral bevel final drives|
|GL-4||Equivalent to obsolete MIL-L-2105 Specification. Usually satisfied by 50% GL-5 additive level or stand alone optimized chemistry||Manual transmissions, spiral bevel and hypoid gears where moderate service prevails. Often used in systems that contain yellow metal|
|GL-5||Part of SAE J2360 Specification (formerly MIL-PRF-2105E). Primary field service recommendation of most passenger cars and truck builders worldwide||Used for moderate and severe service in hypoid and all other types of gears. Also may be used in manual transmissions. Not for use in systems that contain yellow metal|
|GL-6||(Obsolete)||Severe service involving high-offset hypoid gears|
|MT-1||Part of SAE J2360 Specification. Formulated to protect against thermal degradation, wear and oil seal degradation||Non-synchronized manual transmissions, used in buses and trucks. System needs are dependent on chemistry used|
Each automobile manufacturer has a set of unique test requirements for rear axle factory-fill and manual transmission fluids. An SAE J2360 lubricant is a good starting point for the majority of these requirements and is often recommended on axle oils for service-fill. API MT-1 and sometimes API GL-4 lubricants are a good starting point for the majority of these requirements in manual transmissions and can be recommended for service fill.
NOTE – SAE J2360 is equivalent to API GL-5 + MT-1