Imagine training for marathon. You work out, run every day, eat right and give your body the proper amount of rest. But when the marathon day comes, you forget to do one thing: breath. You can guess how well you'd perform. Cars need a constant flow of air just like we do. Aside from fuel, air is the next essential ingredient in keeping your engine running. The air needs to be coming into the engine continually, without restriction and without debris.
Air filters keep all the outside junk like bugs, leaves and dirt from getting inside of your engine. Over time, these filters can get clogged with dirt and need to be replaced. Depending on your driving habits and road conditions, an air filter can usually last a long time. But if the filter is allowed to get too dirty and not doing its job efficiently, it can either starve your engine for oxygen, let in too much junk -- or both.
When you change your oil, take a look at your air filter and see if it looks clean. Replace it if necessary to keep that engine running smoothly and protect it against the all those bugs and other road debris that ends up on your front bumper.