Numbers and Letters

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What do the numbers and letters mean ... using the example SAE 20w/50, API - SJ/CF

SAE 20w/50

SAE which stands for 'Society of Automotive Engineers',  is a viscosity numbering system used to identify the 'thickness' of an oil or it's resistance to flow, Viscosity. Generally, the lower the number, the thinner the oil.

The first part, 20w, is the measurement in this instance to indicate the light end of the product for cold start.

The second part, 50, is the measurement of the high end or heavy end of performance at higher operating temperatures.


API which stands for 'American Petroleum Institute' ‚ is an indicator of the performance level of the lubricant as determined by the type of additive package used in formulating the oil.

The first part, SJ, is a petrol engine oil rating, with the S representing petrol engine and J representing the performance level or additive package type.

The second part,CF, is a diesel engine oil rating, 'C' being diesel and 'F' being the performance level.

Now all of this sounds easy, yes ?, NO, the manufacturers recommendations must also be taken into account as all of the above is a 'General Outline' only, intended to help you understand what the numbers mean.

There are many different Viscosities ranging from 0w/30 to who knows where, and equipment manufacturers generally state what ratings they prefer to see used in their engines, in the vehicles hand book. For more information on API, refer to our API Guide.

All the above relate to engine oils, Gear Oils and Hydraulic Oils are rated differently.

Automotive gear oils whilst in the main are rated using the letters SAE, are of a different type of SAE rating as opposed to engine oils.