This occurs when the filter screen becomes abrasive, worn out at frayed due to metallic particulate matter from the main and return lines through which the oil flows, fly-by particulate matter from sand, wear metal or other debris of the equipment through which oil flows at a high speed. These particles lodge in the filter material over time, blocking the pores, and a pile-on effect created by more such particles results in these pores becoming larger in size. Regular checks of the gaskets and seals can help prevent this. Once again, a Beta Ratio Test can ascertain if there has been an increasing trend in higher-micron particles that are not being captured by your filter. Alternatively, a particle counter can give you a quicker (but less accurate) estimate too.