SAE Viscosity Standard

The Society of Automotive Engineers, Inc. of the United States has established a viscosity classification for oil, and there are various standards for engine oil and gear oil.

Engine Oil Viscosity Standards

The low temperature side stands for Winter with the number W, which is an index for the temperature when the engine is cold in winter. (I will add it later.) In the case of 0W oil, the viscosity is -40%.

The lower the number, the softer the oil is even when the engine is cold and not fully warmed up, so the engine starts better and fuel consumption is reduced.

Viscosity at low temperature

0W  -40℃
5W -35℃
10W -30℃
15W -25℃
20W -20℃
25W -15℃


The notation on the high temperature side is only a number. Except for special engine oil, most of the recent engine oils are multi-grade, soft at low temperatures, and are made so that the viscosity does not decrease at high temperatures. For example, "10W30" is used.

Some manufacturers still produce and sell single grade oils for classic cars and motorcycles.

High temperature side viscosity

SAE Viscosity
High temperature side - Kinematic viscosity cSt@100°C


Therefore, the high temperature side should not be lower than the viscosity grade of the oil specified by the vehicle manufacturer, because if the viscosity is not sufficient, wear will increase significantly.

In summer, 20W will be fine all over Japan, and in winter, 20W will be fine all year round if you are west of the Kanto region, excluding mountainous areas. In winter, 20W should be fine west of the Kanto region except in mountainous areas. 0W should be used in cold regions.

Even if you are not in a cold climate, you can use 0W all year round, but you are sacrificing some performance, such as oil life, lubrication performance, cost, etc. Therefore, you should use at least two types of oil, one for summer and one for winter.

This is a bit of a leap, but in World War II, of course there was no such thing as multi-grade oil, and on top of vehicles weighing tens of tons, Soviet tanks were diesel-powered and had poor starting performance, so they must have used oil that was very hard.

Even today, there are special heaters to warm engines in Russia and Scandinavia.

off topic

The engine pressure can sometimes be surprisingly increased by simply replacing the used oil with new oil of the same brand, This is a test to check for air leakage and compression pressure in the combustion chamber by connecting the hose of a pressure gauge to the plug hole and cranking the engine.

If the engine seems to be losing horsepower and torque as the total mileage increases, it may be a good idea to try one or two grades of harder oil.


Gear oil viscosity standard

low temperature side High temperature side - Kinematic viscosity cSt@100°C
70W -55℃
75W -40℃
80W -26℃
85W -12℃
90    13.5~24
140    24~41




Although the values are different, the readings are the same as for engine oil.

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