Passenger Car Motor Oil Part 2 – SAE Grades & Viscosity
The Society of Automotive Engineers (SAE), a U.S.-based, globally active professional association and standards developing organization for engineering professionals in various industries, established a numerical code system for grading motor oils according to their viscosity characteristics.
SAE viscosity grade numbers refer to two types of measurements:
– One set which measures cold temperature performance (0W, 5W, 10W, 15W and 20W). The W denotes a “winter grade” from the days when oils were only monograde oils.
– The second set of measurements is for high temperature performance (8, 12, 16, 20, 30, 40, 50). The document SAE J300 defines the viscometrics related to these grades. A common grade is 10W-30.
Below’s figure depicts the SAE Viscosity Grades for Engine Oils:Source: Engine Oil Viscosity Classification, J300 Jan2015, SAE at www.sae.org.
Kinematic viscosity is graded by measuring the time it takes for a standard amount of oil to flow through a standard orifice, at standard temperatures. The longer it takes, the higher the viscosity and thus higher SAE code. Bigger numbers are thicker. The SAE has a separate viscosity rating system for gear, axle, and manual transmission oils, SAE J306, should not be confused with engine oil viscosity.
What is Monograde?
Monograde is an oil which is only suitable for use within a very narrow temperature range. SAE monograde oils usually fall into 2 main categories:
a) The grades with a ‘W’ after them (SAE 5W, 10W, 15W and 20W) are only suitable for use in winter time because they are generally thin oils which are good for use in winter or for cold starting.
b) The ‘summer time’ grades (SAE 20, 30, 40 and 50) are more suited for warmer or summer time temperatures or for use in an already hot engine.
The problem with monograde oils is that they can only operate efficiently over a narrow temperature range. For example, a SAE 10W oil is fairly thin, which is good for cold starting but it becomes too thin for effective lubrication as the temperatures rise, the oil gets hotter, it starts to lose its viscosity or thickness. An SAE 40 oil is fairly thick, which provides good lubrication when the engine is hot, but thick oils can cause drag when used from cold (e.g. starting the motorcycle first thing in the morning).
Gear and gear box oils, which are used in engines where the gearbox is separate from the engine, are monograde oils but they use a different range of SAE grades to measure their viscosity.
What is Multigrade?
Multigrade oil is oil that has special polymer additives called viscosity index improvers (VIIs). VIIs are added to the oil to help bring the difference in viscosities for most monograde oil that is too large between the extremes of temperature, closer together. These additives also allow the oil to flow easily under cold and hot conditions.
Multigrade oil is designated with two numbers:
– The first one is followed by the letter “W”. This first number indicates the viscosity (weight) of the oil in Winter (cold)
– The second number indicates the viscosity (weight) of the oil under normal engine temperature (hot). It is important for the oil to be thin enough to flow easily when cold. This protects the engine parts from undue wear in cold starting conditions.
Advantages of Multigrade Oils
- One oil for year-round use
- Improved low-temperature starting
- Excellent high-temperature performance
- Improved overall fuel economy – less idling time and faster warm up
- Less battery drain on cold starts
- Faster, full-pressure lubrication over a wider temperature range
We hope you find this information useful and if you have further enquiries, please consult our distributors in your country or email us at email@example.com for assistance.