A differential pressure transmitter with an orifice is widely used as a major field flowmeter. A major drawback of this flowmeter at operation site is blockage of an impulse line, which connects a differential pressure transmitter with an orifice tap. By conducting experiments at a water line, we have developed a method to diagnose the blockage of the impulse line based on the evaluation of pressure fluctuations; this is installed on a multi-sensing differential pressure transmitter. This paper utilizes this method to diagnose the blockage at an oil line that carries a highly viscous fluid. In particular, we focus on the relation between the amplitude of the pressure fluctuations at the oil line and the kinetic viscosity (1.0 × 10^-5 to 3.0 × 10^-5 m²/s). Through experimental investigations, the following results are derived: (1) The highly viscous fluid in the impulse line does not attenuate the pressure fluctuations transmitting to the multi-sensing differential pressure transmitter. (2) Highly viscous flow in the main line has small pressure fluctuations; however, these fluctuations have the same characteristics of the impulse line blockage as those at a water line. Therefore, the proposed method can diagnose the impulse line blockage at an oil line as well as at a water line.