Which of the following is not an indicator of proper oil burner operation?

Visible soot buildup is not an indicator of proper oil burner operation; rather, it suggests that there are issues that need to be addressed. A healthy oil burner should produce a clean flame with minimal or no soot emissions. A consistent flame color, typically blue or yellow, indicates that the combustion process is efficient and that the burner is operating correctly. Efficient fuel usage reflects optimal combustion where the fuel is burned completely, resulting in maximum energy output and minimal waste. Minimal smoke production further signifies that the oil burner is functioning properly; when combustion is adequate, there should be little to no visible smoke. Therefore, visible soot buildup is a clear sign of inefficiency or malfunction in the burner operation, marking it as the option that does not indicate proper functioning.