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a pressure relief/bypass valve. All the filters had anti drain-back valves (the big rubber flapper plus a coil/leaf spring) but only the Amsoil had an additional pressure relief/bypass valve. With the Amsoil filter part of the oil goes directly through the bypass valve to the engine (without filtration), while part of the oil goes through a more restrictive/efficient filtration. Just for example, suppose the Amsoil filter allows 50% of the oil to pass unfiltered (and no restriction in flow), while the remaining 50% gets filtered down to 1 micron (with a big restriction in flow). The other filter may filter 100% of the oil, but only down to 10 microns with a moderate restriction in flow. When both filters are clean, the pressure drop and AVERAGE filtration efficiency maybe about the same. The tradeoff is whether you want all the oil to be filtered all the time, or allow some oil to bypass, which ensures some oil flow regardless of the condition of the filter.
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