API Classifications



As we saw, the earliest oils were straight mineral oils. However, as technology advanced, various additives were added to enhance performance and it is at this the American

Petroleum Institute (API) was formed to develop a classification for the engine and gear oils.

Engine oils were divided into petrol engine oils and diesel engine oils. Petrol engine oils were given a suffix “S” denoting service station where most petrol cars are serviced. The diesel engines had a suffix “C” denoting commercial as most diesel engines were for commercial use. The lowest quality petrol engine oil was given SA, then SB, SC, SD, SE, SF……..SN, SN representing the highest quality petrol engine oil. The diesel engines were given CA, CB, CC, CD…..CI-4, CJ-4 and now CK-4.

Technology advancement has created oils that meet both petrol and diesel specification such as API CI-4/SL. This type of oil can be used both in modern high performance diesel engines and petrol engines.

Gear oils had the following classification;

  • API GL-1, API GL-2, API GL-3, API GL-4 and API GL-5.
  • API GL-1 is a straight mineral oil used in very old gear boxes but is now obsolete.
  • API GL-3 is an industrial gear oil.
  • API GL-4 is a manual gear box oil.
  • API GL-5 is a differential oil also referred to as a hypoid gear oil or axle oil.

It is common practice to refer the gear oil or differential oil as a CC oil. Gear oils are different from differential oil.



In the old days various base oils were mixed to get the correct thickness (viscosity) required.  Marketing companies would claim their product is better, thicker or thinner for your application depending on the seasons. This confused buyers and there was need to classify the viscosity grades.

The Society of Automotive Engineers (SAE) came up with a classification for winter and summer grades.  The winter grades were 0W, 5W, 10W, 20W. The smaller the number the lower the temperature the oil can stop flowing. Summer grades were 20, 30, 40 and 50 (the lower the number, the thinner the oil). It was common practice then to use a winter grade say 10W during winter but when summer comes the oil becomes too thin and the engine starts smoking. The winter oil is removed and replaced with an SAE 30 if the engine is new or SAE 40 if it is old. Again, when winter comes the oil becomes too thick that it cannot be pumped around the engine. The summer grade is then removed and replaced with winter grade. These viscosity grades are still with us and are referred to as monogrades.

For gear oils the oils for winter were given classification 70W, 75W, 80W, 85W. The smaller the number, the lower the temperature the oil will stop flowing. The summer grades were 80, 90, 140 and 250. Just like the engine oil case, you had to remove winter gear oils when summer comes and vice versa.



Multigrade oils are oils that can be used both in winter and in summer. Over the years, technology developed a product commonly known as motor honey. It was found that if a certain proportion of this material is added in say SAE 10W, you can get a product that behaves like SAE 10W in winter but when summer comes it behaves like an SAE 40.

Such product was classified as SAE 10W40. There are many other versions such as 5W40, 5W30, 20W50, 20W40 etc…

Similarly a material was found that will make an oil flow at much lower temperature that it would have without such a chemical. This chemical was referred to as a pour point depressant. Thus, if this material is added on an SAE 90 gear oil the new gear oil can flow to temperatures that an SAE 80W can flow. This new oil is therefore classified as SAE 80W90. You can similarly make SAE 85W90 or SAE 85W140.