The effect depends on the water content:

• Undissolved (free) water droplets form two separate phases in an oil sample, making it impossible to be correctly analysed by optical particle counters, as this would distort the measurement result (see also here).
  Such samples can be correctly measured using a microscope, like the PAMAS FastPatch 2 GO.
• Slightly cloudy oil samples can be clarified by dilution with a moisture-masking liquid (e.g., Aerosol OT), allowing them to be measured correctly.