• Home
• Maintenance and Motor Oil Advice & Information
• Viscosity / Grade Classifications Explained

Engine, Gear Oil & Grease Viscosity / Grade Classifications Explained

We explain the mechanics behind the different grade classifications of car / motorcycle oils & greases.

Viscosity Classifications

INDUSTRIAL LUBRICANT CLASSIFICATIONS

ISO (International Standards Organisation) viscosity classification. The ISO viscosity classification uses mm2/s (cSt) units and relates to viscosity at 40oC. It consists of a series of 18 viscosity brackets between 1.98 mm2/s and 1650 mm2/s, each of which is defined by a number. The numbers indicate to the nearest whole number, the mid points of their corresponding brackets. For example, ISO viscosity grade 32 relates to the viscosity bracket 28.8 to 35.2 mm2/s, the mid point of which is 32.0 mm2/s. This is illustrated in the table below, which shows the ISO viscosity grade numbers, the mid-points of each bracket, and the viscosity limits

This system is now used to classify all industrial lubricating oils where viscosity is an important criterion in the selection of the oil. Cutting oil and some other specialized products are more important in relation to grade selection.

Kinematic Viscosity at 40°C (mm 2⁄s)

*ISO Viscosity Grade

GREASE CLASSIFICATIONS

NLGI Grease Consistency Classification

The commonly used grease consistency classification is that established in the USA many years ago by the National Lubricating Grease Institute (NLG). This classifies greases solely in terms of their hardness or softness; no other property or performance level is taken into consideration.

The classification consists of a series of consistency range, each of which is defined by a number (or numbers) 000 to 6. The consistency, defined by the distance in tenths of a millimetre, that a standard cone penetrates a sample of the grease number under standard conditions at 25oC. This system is used to classify industrial greases.

NLGI Grease Classification (National Lubricating Grease Institute

AGMA Specifications for Gear Lubricants

The American Gear Manufacturers Association (AGMA) have issued specifications and recommendations for gear lubricants used in various types of gear application. AGMA Standard 250.04 details specifications for rust and oxidation inhibited (R and O) and extreme-pressure (EP) lubricants used in enclosed gear drives. The viscosity brackets correspond to those given in ASTM D 2422 Standard Recommended Practice for Viscosity System for Industrial Fluid Lubricants.

AGMA Viscosity Grades for Enclosed Gearing

Oils marked ‘comp’ are compounded with 3 to 10% fatty material.
The AGMA Standard 251.02 details specifications for three types of open gear lubricants – rust and oxidation inhibited (R and O), extreme-pressure (EP) and residual type gear oils. In this case the viscosity brackets for the higher viscosity grades are measured at 100 C.

LUBRICANT VISCOSITY CLASSIFICATIONS / ENGINE OIL LUBRICANT CLASSIFICATIONS

SAE J300 September 1980 (Engine Oils)
The most widely used system for engine oil viscosity classification is that established by the Society of Automotive Engineers (SAE) in the USA. In this system two series of viscosity grades are defined – those containing the letter W and those without the letter W.
Grades with the letter W are intended for use at lower temperatures and are based on a maximum low temperature viscosity and a maximum borderline pumping temperature, as well as a minimum viscosity at 100C. The low temperature viscosity is measured by means of a multi-temperature version of ASTM D2602 ‘Method of Test for apparent Viscosity of Motor Oils at Low temperature using the Cold Cranking Simulator’. Viscosities measured by this method have been found to correlate with engine speeds developed during low temperature cranking. Borderline pumping temperature is measured according to ASTM D3829 ‘Standard Method for Predicting the Borderline PumpingBorderline Pumping Temperature of Engine Oil’. This provides a measure of an oils’ ability to flow to the engine oil pump inlet and provide adequate engine oil pressure during the initial stages of operation.

Oils without the letter W, intended for use at higher temperatures, are based on the viscosity at 100oC only. These are measured by ASTM D445 ‘Method of Test for Kinematic Viscosity of temperature and Opaque Liquids’. A ‘Multi-grade’ oil is one whose low temperature viscosity and borderline temperature satisfy the requirements of one of the W grades and whose viscosity at 100oC is within the stipulated range of one-W-grades.

Automotive Lubricant Viscosity Grades¹
Engine Oils - SAE J 300, JUne 2001 (DEC. 1999)

*Viscosity (cP) at Temp (°C),Max Cranking 2

**Viscosity (cP) at Temp (°C),Max Pumping 2

***Viscosity4 (cSt) at 100°C Min

*Viscosity⁴ (cSt) at 100°C

**Viscosity⁵ (cP) at 150°C and 10sec^-1

¹ - All values are critical specifications as defined by ASTM D3244

² - ASTM D5293

³ - ASTM D4684. Note that the presence of any yeild stress detectable by this method constitutes a failure regardless of viscosity

⁴ - ASTM D445

⁵ - ASTM D4683, CEC L-36-A-90 (ASTM D 4741) or ASTM DS481

⁶ - 0w40, 5w40 & 10w40 grades

⁷ - 15w40, 20w40, 25w40 & 40 grades

AUTOMOTIVE GEAR LUBRICANT CLASSIFICATIONS

SAE J306 (Gear Oils): Again, the classification is based on the lubricant viscosity measured at low and/or high temperatures. The high temperatures values are determined according to method ASTM D445. The low temperature values are determined according to method ASTM D2983 ‘Method of Test for Apparent Viscosity at Low Temperature using the Brookfield Viscometer’ and are measured in mPa.s (c.P).

Multi-grade oil satisfies the viscosity requirements of one of the W grades at low temperatures and one of the non-W grades at high temperature.
It should be noticed that there is no relationship between the SAE engine oil and gear oil classifications. A gear lubricant and an engine oil having the same viscosity will have widely different SAE grade designation as defined in the two classifications.

Automotive Lubricant Viscosity Grades
Gear Oils - Except SAE J 306, 1998

*ASTM D2983 Temperature °C for Viscosity of 150000mPa.S⁽¹⁾

**ASTM D445(mm)2⁄s Viscosity at 100°C

*ASTM D445(mm)2⁄s Viscosity at 100°C

**ASTM D445(mm)2⁄s Viscosity at 100°C

¹ - Using ASTM D 2983, additional low temperature viscosity requirements may be appropriate for fluids intended for us in light-duty synchronised manual transmission.

² - Limited must also be met after testing in CEC 1-45-T-93, Method C (20 hours).

³ - The precision of ASTM D 2983 has not been established for determinations made at temperatures below -40°C. This fact should be taken into consideration in any producer-consumer relationship.

Note: 1cP = 1 mPa.s; 1cSt = 1mm2⁄s

ISO Viscosity Grades

Viscosity System for Industrial Lubricants

*Viscosity cSt. @ 40°C

Approximate Comparison of Different Viscosity Scales

The following table is for the conversion of viscosities in one system to those in another system at the same temperature.

For higher viscosities, the following factors should be used.

• Kinematic = 0.247 Redwood Saybolt = 35.11 Engler
• Engler = 0.132 Kinematic Engler = 0.0326 Redwood
• Redwood = 4.05 Kinematic Saybolt = 1.14 Redwood
• Saybolt = 4.62 Kinematic Kinematic = 0.216 Saybolt
• Kinematic = 7.58 Engler Engler = 0.0285 Saybolt
• Redwood = 30.70 Engler Redwood = 0.887 Saybol

Note: The first part of the table mark with an * should only be used for the conversion of kinematic viscosities into Engler, Redwood or Saybolt viscosities, or for Engler, Redwood and Saybolt between themselves. They should not be used for conversion of Engler, Redwood or Saybolt into Kinematic viscosities

COMPARATIVE VISCOSITY CLASSIFICATIONS

Note:

Viscosity can only be related horizontally.

Viscosities based on 95 VI single grade oils.

ISO and AGMA viscosities are specified at 40oC.

SAE 5W, 10W, 75W, 80W and 85W viscosities are specified at low temperature. Equivalent viscosities at 100oF and 210oF are shown.

SAE 90-250 (Gear Oils) and SAE 20-50 (Engine oils) are specified at 210oF/99oC.