What should remain on the old filter when changing the oil?

When changing the oil, it is essential that the old filter retains the square seal, commonly referred to as an O-ring or rubber gasket. This seal ensures that there are no leaks between the filter and the engine. During installation of the new filter, it is important to inspect and ensure that this O-ring is not left behind on the engine block, as this could lead to a poor seal, resulting in oil leaks and potentially causing serious engine damage.

The other components mentioned, such as magnetic tape, pressure relief valves, and thread sealers, are not designed to remain on the old oil filter or have specific functions related to the filter's performance in the same context. Magnetic tape is not typically a part of oil filter design. Pressure relief valves are integral to filter operation but do not detach with the filter during an oil change, and thread sealers are not used on oil filters as they are designed to create a seal on threaded connections, which is a different application. Therefore, the focus on the square seal is crucial for maintaining proper engine function and preventing oil leakage.