diesel fuel; diesel oil; gas oil; light oil
The ordinary word for fuel used in diesel engines. English labels vary by context: in everyday use it is usually 'diesel fuel', while 'gas oil' or 'light oil' may appear in petroleum or regulatory contexts.
You put diesel fuel in a diesel vehicle.
I accidentally put in gasoline instead of diesel fuel.
I put diesel fuel into the truck.
This bus runs on diesel fuel.
I refueled with diesel at the gas station.
軽油 is a lighter, less viscous petroleum fuel used in diesel engines, while 重油 is a heavier, thicker fuel oil used mainly in ships and industrial boilers.
A Sino-Japanese compound written with 軽, 'light', and 油, 'oil'. The spelling reflects its classification as a lighter petroleum oil in contrast to 重油, 'heavy oil'; this is a component explanation rather than a detailed historical derivation.