Monday, August 25, 2008

About car engine oil

Whenever we bring our car to workshop for periodic maintenance service, the engine oil will be changed. Here are some of the essential knowledge about car engine oil that is practical to be known by every car owner.

The main function of the engine oil is to lubricate the piston movement, reduce direct contact between moving parts in the engine, and hence decrease the friction, wear & tear and heat produced. Some of its other functions are to clean up the engine, prevent oxidation and corrosion, etc.

There are 3 types of car engine oil available in the market:

  • Mineral oil: is the natural oil refined from petroleum. It needs to be changed every 5,000km.
  • Full-synthetic oil: is artificially synthesized (man-made) and consist of chemical compounds which were not originally present in natural petroleum. It has higher viscosity index than mineral oil, more expensive to produce and hence has a higher price tag too. It provides good protection to engine, able to enhances engine performance and power, and can help in fuel economy savings. It needs to be changed every 10,000km.
  • Semi-synthetic oil (synthetic formulation / synthetic blends): blends of mineral oil with no more than 30% of synthetic oil, to get some of the benefit from synthetic oil with a lower price tag. It provides better engine protection than mineral oil and performs better in reducing wear and tear. It needs to be changed every 7,500km.
The kinematic viscosity of car engine oil is graded with the SAE (Society of Automotive Engineers) gradings. The kinematic viscosity is measured at a reference temperature of 100°C. The higher the viscosity, the higher the SAE grade number is.

Nowadays a multi-grade SAE designation is normally used, which carries 2 numbers separated by a "W" in between. For example, here are the SAE gradings of engine oil used by the Toyota service centres in Malaysia:
  • Full-synthetic: 5W-40
  • Synthetic formulation: 10W-40
  • Premium mineral: 15W-50
The letter "W" denotes "Winter". The number before "W" indicates its cold crank performance. Lower number indicates the oil can be pumped by your engine even at a lower temperature. A 0W oil is well tested at -35°C, a 5W at -30°C, a 10W is tested at -25°C and a 15W is tested at -15°C. The number after "W" is the kinematic viscosity of the oil at high temperature. Oil with lower viscosity number is thiner.

It is advisable that you use the engine oil with the SAE rating as recommended by the car manufacturer. You can find this information in the car manual.

Bear in mind that the SAE grading does not state the quality of the oil. The quality of oil is determined by its API (American Petroleum Institute) rating.

For petrol engine using spark-plug ignition, the API listings start with letter "S" (meaning Service category), and for diesel engine using compression ignition, the API listings start with letter "C" (meaning Commercial category). Higher alphabet position of the 2nd letter denotes a better quality. For example, the SM rating has a better quality than SL. CJ-4 is better than CI-4.

Currently Toyota service centres in Malaysia use SM grade full synthetic oil, and SL grade for both synthetic formulation and premium mineral oil. Their diesel engine oil is CF grade.


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