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Motor oil can be divided into two different types of components – base oil and additives.  Base oil makes up most of a motor oil formulation (typically 70% - 90% of the formula, depending on the motor oil grade).


What is base oil made of? 

 Base oil is derived from crude oil and refined to use in motor oil formulations. Depending on the amount of refining done to the base oil determines the properties and characteristics of the base oil. The refining process removes many impurities in the base oil such as waxy materials and sulfur.  The amount of waxy materials, sulfur and viscosity behavior of the oil at lower and higher temperatures determines the quality or group of the base oil. 


What are the different types of motor oil?

 The properties of base oil groups are defined by the American Petroleum Institute.  The better the qualities of the oil, the higher the group number and the better performance of the oil. 

  • Synthetic motor oil - Motor oil products that use only the highest quality base oils. 
  • Synthetic blend (or part synthetic) - When a mix of high group base oils and lower group base oils are used.
  • Conventional (or mineral) motor oils - Products using lower group base oil.


Is synthetic oil better than conventional?

 In comparing similar viscosity base oils, synthetic base oils will show improved performance over conventional base oil in the following ways:

  • Synthetic oils burn off less (have lower volatility) than conventional oils in high temperature situations such as in an automotive engine
  • Synthetic oils move better at lower temperatures and continue to move at extreme low temperatures that conventional oils would not move
  • Synthetic oils contain less impurities such as sulfur and wax
  • Synthetic oils have been found to cling to surfaces better
Castrol types of motor oil side by side comparing conventional and synthetic base oil

Are engine oil additives necessary?

 But base oil isn’t the only thing that defines the performance of a motor oil.  The remaining components of motor oil are the additives and viscosity modifiers of the formulation.  These components play a critical role in protecting the engine.  Viscosity modifiers change the viscosity or flow characteristics of the motor oil – allowing the oil to flow better at low temperatures and to help keep the oil thick enough at high temperatures to protect metals surfaces from contacting each other.  And the additive package protects the engine in many ways, such as:

  • Reduce deposits from forming on the pistons and cylinders
  • Prevent oxidation and thickening of oil to flow through engine pathways
  • Prevent the formation of sludge and varnish on metal surfaces such as the oil pan, rocker covers and screens
  • Reduce metal to metal contact on key metal surfaces such as the cam shaft and bearings
  • Improve fuel economy
  • Keep engine running at optimum conditions (preventing low speed pre-ignition)
  • Protect seals from leaking or cracking
  • Reduce foaming as the oil moves through the engine
Various types of Castrol® motor oil and viscosity grades

What is oil viscosity grade?

 Viscosity is the resistance to flow of a fluid. Motor oil viscosity grades are based on a scale developed by the API (American Petroleum Institute) and are based on the resistance the oil gives to flowing at two different temperatures – cold and high temperature. The measurement of viscosity at high and low temperatures are properties of multi-grade oils. 


The low temperature viscosity of the oil is a measurement that simulates starting a car on a cold winter day.  That value has the letter “W” after the number and has a dash after the W.  For example, if the oil is a 5W-30, the 5W part describes the viscosity of the oil at low temperatures. The lower the number, the faster the oil will flow at vehicle start up.


The high temperature viscosity is the number after the dash and is related to the viscosity of the oil as it is moving around your engine after the car has warmed up and is at normal engine temperature. In the 5W-30 example, the 30 defines the viscosity of the oil at normal engine temperatures. Again, the lower the number, the lower the viscosity of the oil and the faster the oil will move around the engine.


In recent years, lower viscosity oils are most often required by auto manufacturers.  The key reasons for the need for reduced viscosity oils are engine design and fuel economy. Engine size has reduced over the years while delivering more power.  The smaller size of these engines requires thinner oils to move through engine pathways to keep vital engine parts protected.  And lower viscosity oils move easier which results in better fuel economy in vehicles. 

SAE 5W-30 Grade description

What is the thinnest motor oil?

 Currently, 5W-30 and 5W-20 are the most popular viscosity grades required by vehicles on the road.  But 0W-20 is the fastest growing viscosity grade and is likely to be the most popular grade required by vehicles on the road in the within the next 5 years. And viscosity grades are getting thinner – there are vehicles on the road that require 0W-16, which is the thinnest oil required currently.  In the future, oils may go thinner than 0W-16 as engine design continues to evolve and improve.


If you are wondering what the correct grade of oil is to use for your vehicle, look in your vehicle owner’s manual.  Another great source to find the correct oil for your vehicle is the Castrol Oil Selector.