What is the standard breakdown voltage of transformer oil?

There is no single universal number because breakdown voltage depends on the test method (electrode gap, rise rate), oil condition, and whether the oil is new or in service. Standards such as IEC 60156 define how the test is performed, and utilities then specify acceptance criteria for new oil and service oil based on their risk approach. In practice, higher BDV indicates cleaner, drier oil, but decisions rely on multiple indicators, not BDV alone.