Continuing our series on engine oil basics, let us look at the differences between mineral, semi-synthetic and fully synthetic oils. This is among the most important subject when we want to buy an engine oil so we MUST be well-versed with these facts, especially when there are so many wrong information, misconceptions and myths floating around even among mechanics!

READ HERE: What are SAE grades?

Base oil

The type of engine oil is derived from its base oil, so this is where we will begin this topic. A "base oil" is the engine oil that we buy sans the additives and colouring.

There are several types of base oil, especially on the synthetic side but we will just concentrate on mineral and synthetic to simplify things.

Mineral base oil

As the name suggests, it is sourced from raw petroleum. Raw petroleum cannot be used directly from when it is extracted from the Earth and needs to be refined first

Petroleum is heated in a heating tower. The more volatile (lighter) substances including gas, gasoline, kerosene, etc. float to the top and are extracted. Conversely, the heavier substances are left at the bottom of the tower and these include the base for lubricating oil, bunker oil (for ships), asphalt, etc. This base oil is still not suitable for usage in vehicle engine's, though.

It is then mixed with an additive "package" to obtain the final product's characteristics such as grade, friction and shear protection, grade changes under heat, cleaning, and so forth. So, this is the mineral engine oil we buy in the shop.

Synthetic base oil

Generally, a synthetic base oil is formulated in the laboratory to mimic the characteristics of lubricating oil. As such, it is devoid of the free radicals in mineral oil such as sulphur.

Additionally, the engineers could create a base oil with certain characteristics such as heat resistance, grade maintenance and so forth from the beginning even before it is mixed with an additive package.

There are several sources for synthetic base oils including, but not limited to esters (from plants), petroleum gas to liquid, and oils derived from further refining mineral base oil.

Semi-synthetic oil

This brings us to semi-synthetics. They consist of the mixture of mineral and fully synthetic base oils. The content of fully synthetic base oil is no more than 30%.

Which is better?

Synthetic oils are of course better because they generally last longer, provide better protection and cleaning capabilities, and stay in grade longer. The trade off is their higher price. However, mineral and hence semi-synthetic oils have come a long way due to er... refinements in the refining process and better additive packages.

READ HERE: What are SAE grades?