Engine Oil and its viscosity

For satisfactory lubrication of the engine the oil should possess some functional properties of which viscosity of oil is one of the most important properties, as it brings out the oil’s capacity to lubricate. Viscosity is the measure of oil’s resistance to flow. An oil’s viscosity is identified by its SAE’s (Society for Automotive Engineer’s) number. The thinner and oil, lower its number, e.g. SAE 10 W. The numerical relates to viscosity at particular temperature and the alphabet ‘W’, indicates the oil’s suitability for colder temperature.

With the viscosity index improver, the viscosity increses at higher temperature and at lower temperature it does not increase significantly, thus achieving optimum viscosity at lower and higher temperature. Such oils are called multigrade oils, for instance ‘20 W40’ shows thinness at low temperature and thickness at higher temperature. However, there are other service classification of oil apart from viscosity, developed by API- American Petroleum Institute, which indicates service characteristics. It is graded on a scale from SA (the lowest) to SJ (the highest) for gasoline engines, it is graded on a scale from CA to CG. Both the recommendations for viscosity and service classification can be found on label of the oil container.

As we are extremely affected by the prices, quality and other vagaries of the fuel available, we tend to forget that caring to use correct type of oil is perhaps more important than the fuel. Just like any other subject on cars, the first and the best guide for vehicle owners is to follow very carefully maintenance o instruction manual provided by the manufacturers. If one has questions or is unsure about which product to use, contact the manufacturer of the preferred brand of oil. They are specialists with extensive knowledge of petroleum chemicals to remove or ignorance of good. Of course there are always proficient hands to help you at your own dealer’s workshop.

The quality of oil is determined by SAE viscosity grades and by API service classifications. It should then be matched with the recommendation in the service manual and with the label on the oil container having SAE and API ratings. One can always refer to the specification sheet that gives detail on product performance. This enables one to understand why certain brands exceed by large margin the safety and performance level set by SAE and API standards by a higher quality of oil and viscosity range- something which is not necessary for every car- but yes, for high performance and luxury cars. 

Oils should always be changed at the intervals recommended by the maintenance manual. These days the drain life of an engine oil is anywhere between 5000 to 15000 kms (or even more) or six months to one year, whichever is first. At times, due to unusual circumstances, there may be shorter periods to change the oil especially if one drives short distances and a lot of switching on and off of the engine is involved. The condition of the engine oil can also be felt by the slickness of the oil. But your fingers and thumb will need lot of experience to know that. Your trusted service man can always help you.

It always safe to have the level of oil indicating on the dipstick between MIN and MAX marks. One should check the oil level at least once every two weeks or after every 1000 kms, and specially before a long journey. It is dangerous for the engine to run with low or no engine oil. And remember, the colour of the oil has nothing to do with its ratings or performance, it has nearly cosmetic value.
Change of filters is normally recommended with every oil change considering the impurities of fuel and dusty condition of the country.
It is a good habit to keep engine running at idle for few minutes after it is started. One should not rev the engine. This allows the oil to flows all over the moving parts before any load is placed on the engine. Remember, the maximum wear and tear of the engine takes place when it is started for the first time of the day.

Always purchase replacement oils from trusted and established source. And make sure the oil and the filter is changed in your presence, Recheck the drain pug to know if it is plugged properly.

The care the lubricant manufacturers take in their packaging and sealing indicates the seriousness of the matter.



  • Use only recommended engine oil. It should be in the owners manual and if you are unsure call up the manufacturer or the dealer.
  • Change the engine oil every six months or five thousand kms, whichever is earlier.
  • Make a note of your last oil change. At the time oil change make sure you use the recommended lubricants only. Check the packaging of the oil to make sure it is not spurious.
  • Always buy from recognized dealers and outlets.
  • Change oil filter at the time of an oil change.
  • Never drive your vehicle if the engine oil level is low or it does not have engine oil.
  • Most cars have an indicator for engine oil on the dashboard instrumentation plaster. If it indicates low oil do not drive the car till you refresh the engine oil.


Leave a Reply

Your email address will not be published.