What is semi-synthetic engine oil?

Engine oils are still a vast subject to talk about, choosing the best oil for your engine could be very challenging because you have to be very careful about the many properties that oil can have.

The existing types of engine oil today are full synthetic, semi-synthetic and mineral.

Semi-synthetic oil is a combination of the other two types of oil, full synthetic, and mineral. It practically is mineral oil with synthetic additives added, resulting in a better alternative to regular mineral oil, cheaper than fully synthetic oil and it can be used on almost any engine. They are also called synthetic blends.

Semi-synthetic engine oil properties

There are no specific rules of an initial value for the additives that a semi-synthetic oil has in its composition, so the percentage of the additives that a semi-synthetic has, varies between 20% and 50%, oil manufacturers have their own rules for synthetic additives.

However, engine oil properties depend mostly on the base oil and only a 20% max on the additives used. So, you do not have to trust manufacturers that say the oil has great features based only on additives.


The additives used in the semi-synthetic engine oil are are those who have protective properties for the engine like anti-corrosion additives, cleaning additives that helps the engine against dirt or carbon deposits, and also additives that change the base oil.

These additives affect the viscosity, which is a key point when choosing the oil brand.

Oil viscosity

Viscosity index is characterized by the SAE J300 standard. You can find here more information about engine oils

For example, 5W40 and 10W40 oils have a viscosity in the range of 12 to 16.3 mm2 / s at a temperature of + 100 ° C and a minimum viscosity of 2.9 mPa · s at a temperature of + 150 ° C. At the same time, oils marked 15W40, 20W40, and 25W40 have a viscosity in the range of 12 to 16.3 mm2 / s at a temperature of + 100 ° C and a minimum viscosity of 3.7 mPa s at a temperature of + 150 ° C.

Usually a semi-synthetic engine oil has a lower viscosity than a synthetic

Semi-synthetic oil marking

Semi-synthetic oils are labeled in the same way as mineral or synthetic oils . The contains the SAE, API, ACEA standard and manufacturer’s approval.

According to the SAE oil standard, oils are usually divided into summerwinter, and all-season. They differ in viscosity and temperature characteristics. Winter and all-season oils have the letter W, for example, 5W40 or SAE 5W, and summer oils have only one number that characterizes their viscosity at high temps (for example, SAE 20)

According to the API oil standard, oils are marked with two letters: S and C. S is a petrol engines, C is a diesel engines.

Going a little bit in-depth or the API standards, oils for petrol engines have the letters A, B, C, D, E, F, G, H, J, L, M, N. The closer the letter is to the end of the alphabet, the more severe are the conditions where the oil can be used. Today, the popular classes of oils are SJ, SL, SM and CF, CG, CH, CJ. Labeling is also practiced for universal oil. For example, SL / CH. If the oil is energy saving, it is additionally marked with the letters EC.

As for the ACEA standards, A / B is for petrol and diesel cars, C is for petrol with catalytic converter.

semi synthetic oil markings

More oil properties here

Semi-synthetic oil vs synthetic oil

In terms of comparison between these two types of engine oil, the synthetic oil has more advantages due to its features and used additives

  • Semi-synthetic oil is weaker than a fully synthetic engine oil at high and low temperatures;
  • Fully synthetic oil helps reduce oil consumption and fuel consumption better than a semi-synthetic engine oil;
  • Fully synthetic oil helps the engine on cold starts;
  • Synthetic oil has greater fluidity and penetrating power;
  • Fully synthetic oil is more reliable for the long term, its characteristics last longer, so you don’t need to change it more often.

Can I use semi-synthetic oil in my car?

The short answer is yes, you can use semi-synthetic engine oil, it is a cheaper alternative and suitable for most low-cost or older engines that are not that pretentious. 

Therefore, it is advisable to use it if you do not want to pay more for fully synthetic oil, but you will not benefit from the many advantages that synthetic oil has for the engine, especially in the long run.

Mixing semi-synthetic oil with synthetic oil and mineral oil

You can mix semi-synthetic oil with the other two types of oils, but only if you don’t have other options available, it’s always best to mix engine oils from the same brand and type. Their additives are made using the same technology, so the likelihood of an unpredictable reaction decreases. However, refill the engine with new oil as soon as possible when you decide to mix any type of oil.

On the long run mixing semi-synthetic oil with synthetic and mineral oil can cause an unpredictable chemical reaction and the engine will suffer.

Read more on engine oil mixing

What is the best semi-synthetic engine oil?

There are a lot of brands and oil types on the market, choosing the best one always causes a headache mostly for unexperienced drivers. You’ll find the best answer in your car manual. There you will find important and detailed information about the best oil for your car.

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Castrol 03102 GTX High Mileage 5W-30 Synthetic Blend Motor Oil

As for advantages, it has good detergent and protective properties. Among the disadvantages, it has a high rate of decrease in base number, that is, a rapid loss of performance properties. So you’ll need to change it more often.

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ir?t=pexu21 20&language=en US&l=li3&o=1&a=B00AM1Z67Oir?t=pexu21 20&language=en US&l=li3&o=1&a=B00AM1Z67OValvoline High Mileage with MaxLife Technology SAE 5W-30 Synthetic Blend Motor Oil

Great deal, especially for high mileage engines. It helps prevent engine wear and carbon or soot buildups

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Motorcraft SAE 5w30 Synthetic Blend Motor Oil

Another good choice for the engine, especially if your engine supports the 5w30 engine. I have tested this brand myself and it has good overall performances for the engine.


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