When Russia invaded Ukraine, Western sanctions targeted Russian exports, like refined oil. But in the eight months following the invasion, an oil tanker loitering just outside Maltese waters sold nearly two million barrels of Russian oil to Nigerian importers. Blended with products from elsewhere, it was then sold as oil from Malta. It also sold at prices 24% higher than unblended Russian oil.

Done dirty: Nigerians pay a premium for super dodgy fuel

To dodge sanctions, oil companies blend Russian oil between ships – offshore, out of sight. This can lead to dirty fuel. The kind thought ‘good...