As explained to me by an old mechanic back in the 60's-the bypass filter system wasn't moving the volume of oil through the filter that a fully filtered system did. A lot of times on the older cars filter manufacturere (WIX, WGB) would discontinue certain filter cartridges because of a lack of demand for specific sizes. In those cases where the owner continued to drive the vehicle, out of necessity, the toilet paper, and even sanitary napkins were substituted as the filter cartridge because the cartridge was no longer available. I don't know what the filter efficiency was for these "substitue" filters, but I do know that my 53 Chevy car that has been in my immediate family since brand new, was running the sanitary napkin replacement from the late 60's up until about 2 yrs ago when an actual replacement for the sock type filter became available. Granted this car is run infrequently as a restoration project. The only difference I notice is that its easier to go to an online parts supplier for classic vehicles than it is to go to a supermarket to purchase a box of the old syled napkins.