marine lubricants information bulletin 6 - viscosity classifications - viscosity index (VI) and viscosity classification systems

Oct 13, 2026

Upload by hdkawk

Marine lubricants information bulletin 6 explains how lubricating oil reduces destructive wear by maintaining a separating oil film, and shows that viscosity governs the thickness of this film. It defines kinematic and absolute (dynamic) viscosity with common units, then introduces the viscosity index (VI) as a measure of how viscosity changes with temperature and oil composition. It further summarizes SAE J300 viscosity grades for marine and automotive use, outlines ISO 3448 for industrial lubricants, and discusses causes of viscosity changes in used oils.

LanguageEnglish

CategoryGeneral

FormatPDF

Pages3

Reading Time1 min

AccessFree to download

Viscosity and lubrication principles

Why viscosity matters for wear reduction

Kinematic and dynamic viscosity units

Viscosity index (VI)

VI scale and reference oils

Temperature sensitivity and application relevance

Viscosity classification

SAE J300 grades and W vs non-W ranges

Monograde vs multigrade marine oils

Industrial oils: ISO 3448 system

Used oil viscosities

Viscosity increase and decrease causes

marine lubricants information bulletin 6 viscosity classifications The first and most important task of lubricating oil is to keep moving metal parts separated from each other, thus avoiding metal-to-metal contact, which leads to destructive wear. Even finely machined metal surfaces have a certain roughness. Contact of these minute metal projections should be minimized; however, some contact always occurs and results in normal wear of the metal surfaces. If contact is not minimized, heat is generated when the metal parts touch. The heat causes local welding and transfer of metal, which creates scuffing or seizing of the equipment. These actions are called adhesive wear. The oil property that governs the thickness of the separating oil film is the viscosity. Viscosity The commonly used kinematic viscosity is defined as a measure of the restrictive flow of a fluid under gravitational force. The “cgs” unit of kinematic viscosity (one centimeter squared per second), is called one stoke (St) . The SI unit for kinematic viscosity is one meter squared per second and is equivalent to 10,000 St. Usually, centistokes (cSt) is used (1 cSt = 0.01 St = 1 mm2/s) . The absolute or dynamic viscosity is equal to the kinematic viscosity, multiplied by the density of the fluid. It is usually expressed in centipoise (cP) (1 cP = 0.001 Pa.s) . Viscosity index (VI) The viscosity of lubricating oil changes with temperature and the rate of change depends on the composition of the oil. Naphthenic base oils change more than paraffinic base oils. Certain synthetic lubricants change much less than paraffinic oils. To assess this lubricating oil property, the American Society for Testing and Materials (ASTM) created a method to provide a number called the Viscosity Index (VI) . The VI correlates the amount of viscosity change for a given oil, compared to two reference oils having the highest and lowest viscosity indices at the time the VI scale was first introduced (1929) . A standard paraffinic oil was given a VI of 100 and a standard naphthenic oil a VI of 0. Figure 1 shows the relationship between viscosities at 40°C and 100°C. The method has been updated and revised several times to include VI values higher than 100. Figure 1: Schematic Representation of Viscosity Index 0 VI scale 100 40°C 100°C A low VI means a relatively large viscosity change with temperature and a high VI denotes a smaller change of viscosity with temperature. Hence, the VI of an oil is important in applications where an appreciable change in temperature of the lubricating oil could affect the startup or operating characteristics of the equipment. Deck machinery and emergency equipment are examples of typical applications onboard ships. Viscosity classification As the selection of the proper viscosity grade is extremely important, various viscosity classification systems have been developed over the years. The viscosity classification for engine oils was developed by the Society for Automotive Engineers (SAE) in 1911. After many revisions and updates, this classification system is still in place. The current SAE J300 Viscosity Classification is shown on the next page in Table 1. SAE grades 0W through 25W, where W stands for winter, have a maximum viscosity specified at low temperatures (–5°C through –35°C), to ensure easy starting under low temperature conditions, and a minimum viscosity requirement at 100°C to ensure satisfactory lubrication at the final operating temperature. Only SAE grades 20W through 60W have limits set at 100°C, because these grades are not intended to be used in low temperature conditions. For marine applications, monograde oils (i.e., oils without the addition of VI improvers of SAE 30 or SAE 40) are used because of the steady operating conditions in a ship’s engine room. Conversely, automotive oils are normally formulated by adding VI improvers to provide multigrade performance and thus deliver excellent tempera

FAQs

Why is viscosity considered the key lubricating oil property?

Viscosity governs the thickness of the separating oil film that keeps metal parts separated and reduces metal-to-metal contact and destructive wear.

What are kinematic viscosity and dynamic (absolute) viscosity?

Kinematic viscosity measures a fluid’s restrictive flow under gravitational force and is commonly given in cSt or St. Dynamic (absolute) viscosity equals kinematic viscosity multiplied by the fluid density and is typically expressed in cP.

What does the viscosity index (VI) indicate about temperature effects?

VI correlates the amount of viscosity change with temperature compared to reference oils. A low VI means a larger viscosity change with temperature, while a high VI means a smaller change.

How do SAE J300 and ISO 3448 relate to viscosity classification?

SAE J300 provides viscosity grades for engine oils (including W grades for low-temperature limits and non-W grades for high-temperature lubrication). ISO 3448 provides a worldwide viscosity classification system for industrial lubricants based on progressively higher viscosity grades.

Oct 13, 2026

Upload by hdkawk