Should You Choose Semi or Fully-Synthetic Oil?

By: Lisa Harper

What is fully synthetic oil?

Fully synthetic oil is derived from the same crude oil as mineral oil, but the way fully synthetic oil and mineral oil are processed is completely different and causes significant differences in how they perform.

Fully synthetic oil is refined, distilled and purified to a level which enables individual molecules within oil to be tailored to meet the demands of modern car engines. Due to this extensive process fully synthetic oil contains fewer impurities, is freer flowing and performs better under extreme temperatures than conventional mineral oil. Fully synthetic oil also degrades at a slower rate than mineral oil so continues to perform at its best for longer.

Fully synthetic oils are often necessary in newer vehicles with turbocharged engines as they operate at much higher temperature than standard engines. However fully synthetic oil is also often recommended for vehicles that are frequently in start/stop traffic, regularly tow heavy loads or that operate in extreme hot or cold conditions.

Book an oil change
semi or fully synthetic oil

 Benefits of fully synthetic oil include: 

  • Fully synthetic oil contains fewer impurities than mineral oil so is more environmentally friendly and helps to cut engine emissions.

  • Because fully synthetic oil is freer flowing it is able to start circulating as soon as the engine is started whereas with conventional oil it can take a short time to heat the oil up a little and then it starts to circulate. This means that synthetic oil is able to offer protection to your engine parts more quickly than mineral oil which helps to reduce wear. What’s more, as synthetic oil can get to work more quickly it enables the engine to reach peak operating efficiency faster which can help to improve fuel economy.

  • Fully synthetic oil retains its wear protection for a longer time, particularly in extreme heat whereas conventional mineral oil is prone to breaking down and evaporation.

  • A study by the American Automobile Association (AAA) found that engines with fully synthetic engine oil performed on average 47% better than mineral oil in a number of standardised tests. So whilst fully synthetic oils are more expensive to buy initially their increased longevity makes them more cost-effective in the long run.

Regardless of oil type (fully synthetic, semi-synthetic or mineral) the majority of oils currently on the market contain additives to help to alter and improve their performance to provide optimised engine protection.

What is semi synthetic oil?

Semi Synthetic oil is a mixture of fully synthetic oil and conventional mineral oil. Semi synthetic oil can achieve higher levels of performance than from mineral oil alone.

Semi-synthetic oil is more wallet-friendly than fully synthetic oil as it comes at a reduced cost whilst still giving you some of the benefits of full synthetic oil.

can you mix semi and fully synthetic oil

Can you use semi synthetic oil instead of fully synthetic?

We would always recommend sticking to your manufacturer’s recommendation on the type of oil to be used.

If you do use semi-synthetic engine oil instead of fully synthetic oil then it is unlikely to cause any drastic or immediate problems. However semi-synthetic oil will not offer your engine the same levels of protection to your engine that fully synthetic oil does and may result in reduced engine performance. So if fully synthetic oil is recommended then its best to stick to this.

In contrast, engines requiring semi-synthetic oil often benefit from increased protection and performance if you use fully synthetic oil instead of semi-synthetic oil.

Can you mix semi and fully synthetic oils?

As a general rule of thumb, we don’t recommend mixing different oils. We always recommend using the oil specified by your car manufacturer. When you book an interim service with us, we change your oil and always use oil recommended by your manufacturer.

However nothing detrimental will occur if you mix synthetic oil with either semi-synthetic or mineral oil. If this was your only option, mixing oils would be better than running out of oil and potentially causing damage to your engine.

Whilst mixing oil is unlikely to be detrimental it will cause your oil to be compromised and will reduce its ability to perform at its best and it may not last as long as it would have normally.  Should you choose semi or fully synthetic oil?

As a general rule, we recommend following your manufacturer specifications regarding which oil to use. However it’s worth considering using fully synthetic oil if you are driving a lot of miles, are frequently in start/stop traffic, regularly tow heavy loads or live in a country with extremes of temperature. This is because in these circumstances fully synthetic oil is likely to offer more comprehensive protection for your engine and better value for money.

Please note we record all our calls to ensure that we give you the service you deserve.