What does a high differential pressure across an oil filter indicate?

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High differential pressure across an oil filter shows increased flow resistance caused by a clogged filter element. As the filter collects contaminants, its pores become blocked, so the oil has to push through more tightly packed media. That buildup raises the pressure on the inlet side relative to the outlet, signaling the filter is loading up and should be replaced to restore proper oil flow and protection for the engine. While very cold oil can raise viscosity and slightly increase pressure drop, a high differential pressure most directly points to a restricted, dirty filter. A failing oil pump usually lowers overall oil pressure, not specifically the filter’s differential pressure, and a filter that’s filtering with no change would show normal, not elevated, delta-P.