Any filter has minimum particle size that the filter media will NOT filter out. Those small particles will go on through into the engine. Typical filters, like a NAPA Gold 1515, will let particles in the range of a bit under .001" through. (21 microns is the spec.) That is 1/2 to 1/3 the size of standard bearing clearances so can easily scar or tear up or get embedded into a bearing surface, or scar a metal surface.