What are "Copper Corrosion" values and when to worry about them?

A "Copper Corrosion" value is a series of numbers and letters (1A, 1B, 2A, 2B and so on) allocated to a lubricant which allows us to identify how much free Sulphur or active Sulphur is present. The more free Sulphur is present in the oil, the greater it's EP (Extreme Pressure) performance is. This free Sulphur forms a sacrificial layer on metal surfaces and protects the metal underneath. It however does have its drawback and shouldn't be used in yellow metal applications due to corrosion.