Blended or Synthetic Oil: The Best of Both Worlds, or a Halfway House?

What are the advantages of using synthetic oil over blended, and is it worth it in the long run?

Oil is a crucial component of your car's engine. It's pumped around the inside of your engine block and reduces wear and tear. Engine oil prevents metal-on-metal friction, so it’s essential that you use the correct one. Of course, without oil, an engine would seize, so keeping it topped up is essential.

This is also why you should change your oil regularly. Leftovers from combustion and tiny metal shavings all end up contaminating the oil. And, over time, the additives found in engine oil become less effective too, meaning fresh oil is required.

But every time you decide to replace your car’s engine oil, you may have noticed the different types on offer. Two of the most prominent types of oil are blended (or semi-synthetic) oils and fully-synthetic oils. What’s the difference between the two? Read on, to find out!

Oil Explained

Oils typically fall into one of two categories: mineral oils (also known as conventional oils) and fully-synthetic oils. As the name implies, blended oils mix elements of both of these oils to create an intermediary form of engine oil. To understand why, we need to break down what oil is, and how it’s composed.

Conventional oil is derived from crude oil. Crude oil is what comes out of the ground in an unrefined form. Crude oil then goes through a process known as refinement. From that, we derive many oil-based products, including petroleum gas, diesel and, of course, automotive lubricant.

That lubricant makes up what is known as base oil. Base oil counts for 75-90% of the composition that makes up conventional motor oil. The remaining 10-25% is made up of additives. These additives are closely-guarded trade secrets but mostly consist of components that help modify the oil for use in motor vehicles. This includes chemicals that help aid lubrication, prevent wear and tear, and inhibit corrosion.

Synthetic oils, on the other hand, swap out the base oils from crude oil for a heavily-modified petroleum-base (although still derived from crude). The base is chemically engineered from the ground up and is considered man-made: designed specifically for automotive engine applications.

Why Blended or Semi-synthetic Oil?

Blended or semi-synthetic oils combine both mineral oil bases and synthetic ones. Typically, the amount of synthetic oil added is quite low. The key to understanding why blended oils exist is primarily down to cost. Fully-synthetic oils are traditionally more expensive than conventional mineral oils but have much better qualities.

By mixing the two, blended oils can remain cost-effective, while also having some of the qualities of fully-synthetic oils. Such qualities include enhanced viscosity and wear resistance at higher temperatures. Synthetic-blend engine oils can also offer better performance at lower temperatures.

Can I Use Semi-synthetic Oils?

A general rule of thumb is that if your car was designed for, or your manufacturer recommends that you use semi-synthetic oils, then yes. But you should not resort to using conventional mineral oils.

But, if your car was designed for use with full-synthetic oil, using blended oils could prove detrimental. This is because fully-synthetic oil has a lower viscosity.

In layman's terms, this means it flows faster and can pass through small holes and tight gaps easily. Most modern engines have much finer tolerances than those of old, meaning that they require low-viscosity oils for proper lubrication. Using an oil with a higher viscosity (thicker and slower) could have a lasting detrimental impact on your engine.

Can I Use Fully-Synthetic Oils?

Yes, you can. As synthetic oil has been engineered to very specific specifications, it doesn’t rely on the chemical attributes that the mineral or blended oils do. That said, as viscosity is typically lower in fully-synthetic oils, it’s important to note that it can present some issues for older engines.

This isn’t because fully-synthetic oils are particularly harmful to any engine; synthetic oils are predominantly the best solution as they flow easily and are less likely to cause engine sludge. But as seals can crack over time, you'll find that oil with lower viscosity can seep through these crevices with more ease. This will give the illusion that the synthetic oil has caused damage when in actual fact the damage was always there— it’s that the low-viscosity oil highlights this.

But Semi-synthetic Oil is Still Cheaper…

Yes, and one of the best reasons for using blended oil is that it provides the best of both worlds: superior oil attributes at a lower cost. If you're fine with your car continuing to run on synthetic oil, or if you prefer to save money and don't see yourself holding on to it for much longer, then, by all means, continue to use blended oil—you could do a lot worse!

But if you’re keen to extend the life of your car’s engine, or simply want the best for it, then a synthetic oil will outperform a blended oil every time.

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