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Not all motor oils are the same, but you may not know that at the outset. Learn how synthetic engine oil compares vs conventional and semi-synthetic blends.

What Is The Difference Between Conventional, Synthetic, Semi Synthetic Oil?


Motor Oils are formulated and marketed as conventional, full synthetic, part synthetic and semi synthetic.  What is the difference between synthetic and semi synthetic or semi synthetic and conventional?  The difference lies in the type of base oils that are used in the formulation and this is what defines these motor oil categories.


What Is Conventional Motor Oil?

The base oil used in a conventional motor oil is 100% mineral oil which is obtained through the refining of crude oil.  Here the crude oil is distilled and cleaned up to produce a high-quality base oil. This is done without substantial molecular changes being made to the hydrocarbons in the base oil during the process.   


What Is The Difference Between Synthetic And Semi Synthetic Oil?

A synthetic base oil is derived though the creation of new base oil components (synthesis) to produce a stronger performing base fluid.  

  • For a full synthetic motor oil 100% of the base oil composition contains these stronger synthesized base oil components.
  • For semi-synthetic or part synthetic motor oils (these terms are largely used interchangeably) the base oil composition is a combination of synthetic and conventional base oils.  There is no definition of how much synthetic needs to be included in a semi or part synthetic motor oil.  

The use of synthetic base oil can substantially improve the performance of the motor oil.  Synthetic base oils

  • Improve low temperature performance
  • Provide higher thermal stability or heat resistance
  • Possess better oil burn of performance or lower volatility
  • Enable lighter viscosity motor oils such as 0W-20 to be formulated
  • Can help improve motor oil shear stability or the ability to resist viscosity breakdown


What is better semi synthetic or full synthetic oil?

The more synthetic base oil contained in a motor oil in general the better the performance.  In turn, a full synthetic motor oil formulated with a robust additive system will always be better than synthetic blends.  That said a properly formulated semi-synthetic can offer performance benefits over a conventional motor oil. 

How long does semi-synthetic oil last?

Adding synthetic base oil to motor can produce a stronger motor oil due to the benefit of synthetics.  Depending upon the amount of synthetic in the base oil composition the semi-synthetic motor oil can be more resistant to the various factors that can degrade a motor oil.  Castrol incorporates a material amount of synthetic in our part synthetic formulations to help improve oil burn off performance as well as low temperature flow.  That said a robust additive system is also critical to the overall performance of the motor oil.  Thus, one must consider both in judging performance.  While the incorporation of synthetic can produce stronger motor oils Castrol always recommends that one follow the vehicle manufacturers recommendation for oil service and engine oil drain intervals.