According to ASTM D7450, the GL-5 requirement for copper strip corrosion is a rating of 3 when tested for 3 hours at 121°C. The designation of "a" or "b" is not specified.
Most ISO 220 non-EP oils have a rating of 1a or 1b when tested for 3 hours at 100°C. The higher test temperature for the GL-5 qualification test makes it a more severe test than the generally used temperature of 100°C. Different test temperatures are likely to be use specific, automotive versus industrial.
Regardless of the test temperature and despite what the GL-5 standard specifies ASTM D130 performance requirements to be, ASTM D130 results fall into the following classifications and designations; 1a, 1b, 2a, 2b, 2c, 2d, 2e, 3a, 3b, 4a, 4b, 4c. 3A is described as a magenta overcast on a brassy strip. 3B is described as multicolored with red and green showing (peacock), but no gray. 1a is described as light orange, almost the same as a freshly polished strip. 1b is described as dark orange.
Regardless of the performance limits specified by the standard, realistically, a rating of 3 does indicate copper corrosion by the lubricant.