Crude oil quality
The main quality parameters of crude: how light the oil is and how much sulphur it contains.
How to read the number
Light sweet crude yields more light products for less refining effort, and that is what the premium in its price pays for.
When the metric lies
Quality is fixed by geology and changes slowly. So a shift in the differential at unchanged quality is telling you about logistics and demand, not about the oil.
Also known as: density and sulphur content