Do you need to replace drain plug every time you change oil?

 

The factory spec instructs to replace the drain plug each oil change. Most will do that to maintain the integrity of the car's service. The technical reason is the seal. It is captured (not individually replaceable) on the drain plug. It works like a crush washer. Since it is soft, it will deform and seal under torque.

The mechanical theory is - once a crush washer is deformed, it becomes questionable that it will seal a second time. Other than that, the plug itself has plenty of threads and plenty of thread contact with a slight interference fit. I don't see any possibility a reused plug will work loose when torqued properly, just a higher risk the seal might leak.