The oil drain plug is a bolt in the oil pan that doesn't fasten any components, but merely serves as a plug so that oil changes don't require an oil pan removal.
Some oil plugs, such as the one pictured at the right, include a magnetic tip which captures metal particles suspended in the motor oil, which can be inspected at the time of the oil change. The magnet can only capture a little bit of material, however, and should not be counted on for all the particulate filtration. The magnet is often neodynium, since it has to be small enough to fit at the tip of a bolt but strong enough to pull on tiny traces of metal.
Oil Drain Plug Gasket
Many oil drain plugs have a washer-shaped gasket which does most of the sealing when the bolt is tightened. The gasket is often rubber/silicone, but could also be nylon, copper, or aluminium, depending on application or the installer's preference.
Leaking Oil Drain Plugs
Sometimes oil leaks out of an oil drain plug because the plug doesn't seal properly. This could come from a poor flange seal, damaged/missing gasket, or damaged threads on either the bolt or the oil pan.
Sometimes the leak is not due to thread damage, and can be fixed by installing a new washer gasket or using oil plug sealant, such as the one shown at the right. It is not recommended to use a gasket maker as a sealant, because of the time required to properly cure and how easy it is to apply it unevenly, leading to a future leak that seems to be a fix for the leak initially.
Leak-fixing motor oil additives may help in ideal circumstances, but have other drawbacks, and the effects are often lost after the next oil change. Some may soften the drain plug gasket, which could make the leak even worse.
A leak can also occur if the wrong drain plug gasket material is used, but this is uncommon. Generally, materials that start out softer are better at sealing, but have a shorter life span.
Some companies offer alternatives to the simple oil drain plug bolt to potentially make life easier, such as oil drain valves that screw in to the plug location and can be opened without a wrench.