Expert advice: should I choose synthetic or semi-synthetic oil for my car?

Synthetic oil

Back in the day, oil was oil. It came out of the ground and as long as it was the right viscosity for the engine, we’d trust it to do the job. Now there are effectively three kinds of oil: mineral, synthetic and semi-synthetic. And I’m frequently asked: “Should I use synthetic oil for my car?”

I’ve already spoken about the grade of oil drivers should choose for their cars. But once you’ve decided on that, what are the benefits of synthetic oil?

What is synthetic oil?

Both synthetic and traditional mineral oil start life as the crude oil we hear so much about when fuel prices are about to go up. And both have to be refined before they can be used in an engine. But the process for synthetic oils is much more sophisticated.

Scientists working on synthetics go to much greater lengths to remove impurities than with conventional mineral oil. This sees them tailoring individual molecules within the oil to suit modern engines.

Why does synthetic oil make a difference?

Engines place many more demands on their lubricant than they used to. The modern car engine is controlled by computers requiring different behaviour for different types of driving. Synthetic oils have been developed to cater for these very exacting pressures.

They flow much more freely. This enables them to course through the engine the moment the key is turned. That means they’re protecting vital components all the time. With mineral oils that isn’t necessarily the case.

What are the benefits of synthetic oil?

According to oil firms, synthetic oils give improved fuel economy. That’s because they move around the engine more freely as the engine warms up. This helps the engine reach peak operating efficiency more quickly. They put less strain on the battery at start-up because in low temperatures they don’t thicken up too much.

Synthetic oil

Engines have many moving parts. Synthetic oil helps keep them in good nick

Synthetic oils are also cleaner and help reduce engine exhaust emissions. The reduction in impurities means less sulphur and fewer of the other undesirable contaminants that aren’t removed from mineral oil during the refining process.

Will I notice any benefits while driving?

Yes, you will. If you’ve been driving for a while the chances are you’ll have driven a car with an engine that seemed to rattle a lot when it was started from cold. This is because when a car is parked overnight, oil drains into the sump at the bottom of the engine. When the car is started the following morning there is no lubricant at the top of the engine, hence the rattling. And believe me, that noise is as bad for the engine as it sounds!

Semi and fully synthetic oils have an additive blended into them called a friction modifier. This helps the oil defy gravity. When the engine is stopped the oil stays at the top, coating the surface of the vital engine components, and ensuring they don’t rattle on start-up in the morning.

Is synthetic better?

We usually think of real as being better than fake. But talk to any oil company and they’ll say synthetic oils offer improved performance compared with the real thing. Synthetics protect the engine better as it warms up and they’re more stable when the motor is up to temperature. And that’s on top of the benefits I’ve outlined above.

What about semi-synthetic?

Here we get to the downside of synthetic oils: they’re expensive. And that’s why oil companies have developed semi-synthetic oils. They have many of the benefits of synthetic but they aren’t as costly.

Which oil should I choose?

It really depends how old your car is. An older model from the 1980s or early 1990s may not need synthetic oil. Assuming your car is slightly more modern, the chances are you will only have the choice of synthetic or semi-synthetic. I would advise going for the best you can afford. But whichever you choose, make sure it’s the grade and type recommended by your car manufacturer in the user manual.

motorway hard shoulderNick Reid is head of automotive technology for Green Flag and is a fellow of the Institute of the Motor Industry

5 comments on “Expert advice: should I choose synthetic or semi-synthetic oil for my car?

  1. Dave Ferrie 07/03/2018 10:08 AM

    I didn’t know synthetic oil was derived from mineral oil. I thought it was like the old Castrol racing oil which I was told was a kind of vegetable oil ?

    Thanks for the very interesting info.

    • Keith Brown 08/03/2018 10:48 PM

      The clue is in the name – Castrol. It was in fact an oil pressed from the castor oil plant and still has uses in high revving race engines and model aircraft glow plug engines. It isn’t really suitable for general automotive use because it gums up easily and forms varnishes on bearing surfaces, it’s only because race engines are regularly stripped and cleaned that it can be utilised as a lubricant.

  2. Vic Allum 07/03/2018 11:35 AM

    Very informative, As an old mechanic we used to use detergent oils to flush out ,many of the carbon deposits in the engine. Are detergent oils still required for diesel engines. Not to say I have one, just curious !

  3. Philip Southam 16/03/2018 2:49 PM

    No, is the answer. If you run a new diesel car on synthetic oil, the benefits are constant – good economy as well – and although the oil change intervals are extended (being an aging engineer, this does go against the grain), it’s the best way to ensure top performance and longevity. My car is now 48000 miles and purrs like it’s new.

  4. Dever Sanderson 07/07/2023 2:30 PM

    In summary, if you want the highest level of performance, extended oil change intervals, and better protection for your engine, synthetic oil is the superior choice. However, if you’re looking for a more cost-effective option that still offers improved protection over conventional oil, semi-synthetic oil can be a suitable compromise. Ultimately, it’s essential to consider your driving conditions, manufacturer recommendations, and budget when making your decision.

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