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 Think about the last time you filled up your car or boarded a flight. Somewhere in that fuel's history is a refinery engineer who had to pick up a barrel of crude oil.

For decades, a surprisingly large number of them have quietly picked Iran's oil, sanctions or not. The reason has nothing to do with politics. It is pure chemistry.

WHAT IS INSIDE A BARREL OF CRUDE OIL?

Crude oil is not one uniform liquid. It is a mixture of thousands of hydrocarbon molecules, which are simply chains of carbon and hydrogen atoms bonded together.

Short chains are light and produce petrol and jet fuel when refined. Long, heavy chains are sluggish and expensive to break down, ending up mostly as low-value asphalt or fuel oil.

An Iranian man points to an oil production platform at the Soroush oil fields in the Persian Gulf. (Photo: Reuters)

When crude enters a refinery, it passes through a tall distillation tower and gets sorted by boiling point. Light molecules rise to the top and become premium fuels. Heavy ones sink. The refinery's job is to maximise what comes out at the top.

The two most important quality characteristics of any crude oil are density and sulphur content.

Crude oils that are light and sweet, meaning low in sulphur, are usually priced higher because petrol and diesel can more easily and cheaply be produced from them.

An Iranian man works on an oil production platform at the Soroush oil fields in the Persian Gulf. (Photo: Reuters)

Density is measured using API gravity, a scale developed by the American Petroleum Institute.

Think of it like this: water has an API gravity of 10. Anything above 10 floats on water and is considered oil.

The higher the number climbs above 10, the lighter the crude. API gravity ranges from heavy, meaning high density, at less than 25 degrees, to light, meaning low density, at greater than 35 degrees.

Iranian Light sits between 33 and 36, right at the boundary between medium and light. That is the sweet spot most refineries prefer.

Iranian Light crude, with an API gravity of 33 to 36 degrees, converts nearly 70 per cent of each barrel into premium fuels, without the expensive extra processing that heavier crudes demand. (Photo: X/@shanaka86)

WHAT MAKES IRANIAN CRUDE SO HARD TO REPLACE?

Iranian Light, the country's flagship export grade of crude oil, has a sulphur content of 1.36 to 1.5 per cent. Low-sulphur crude is called sweet; high-sulphur crude is called sour. Its distillation yield is roughly 20 per cent light fractions and 50 per cent middle distillates.