In an oil mill, the most commonly used filter press is the discontinuous type pressure filter. This type of filter press is used to separate solid particles from liquids under pressure. It works by applying pressure to force the liquid through a filter medium, leaving the solid particles behind. The filter press operates in batches, meaning it works in a discontinuous or intermittent cycle.
Continuous type filters, on the other hand, are used in situations where the filtering process needs to be continuous, but they are not as commonly used in oil mills. Discontinuous type filters are more suitable for the specific requirements of oil extraction, where the volume of oil to be filtered is not constant and may vary over time.
Thus, the correct answer is option (3): The filter press commonly used in oil mills is the discontinuous type pressure filter.