Suggest you retrofit with a 'dry' type of pleated paper filter; the larger the surface area you can apply the better for 'pressure drop' considerations.
The 'oil-bath' types are only efficient in particle removal at a very narrow 'design point' of specific air flow, and rapidly lose efficiency on 'either side' of air flow that is optimal for the design. These oil-bath types also do not have good efficiency of particulate removal in the 'smaller' µM range .... the smaller the airborne particle, the exponentially more particles will be in 'nature'. Oil bath type 'filters' depend on 'centrifugal force' of particles with significant mass to exit the airstream .. to come in contact with the oil; the smaller the particle (mass and size) the less the centrifugal force generated to 'exit' the airflow to 'impact' into the oil.
Typically in one cubic foot of 'clean' atmospheric air there will be ~10,000 particles at 0,003-0,1 µM; particles at greater than 10µM are 'few' in normal atmospheric conditions.
Small particles will have an additional 'brownian motion' (vibrations) in air/gas and the amplitude of this 'vibration' within a filter media allows the particles to be captured (the amplitude of vibration is larger than the 'pore' of the filter); in liquid (such as in an oil bath-type of filter) there is essentially NO brownian motion involved in particle capture. A 'dry' filter is able to capture ~10-15X smaller particles than when 'wet', even when using the same filter material.
The 'oil bath' types of filters stay 'cleaner' because simply they arent very efficient in capturing 'small' particles and only reach their maximum efficiency at only one distinct amount of air/gas flow.
Use a 'dry' and 'large surface area' filter for *best* particle retention.
