Damaged valve cover gasketBy far, one of the most common causes of an oil leak is a damaged valve cover gasket. These are the rubber seals that sit between your engine's cylinder head and the cover that protects the valve train's upper components. The gasket is responsible for containing oil and stopping leaks, preventing debris or dust from entering the cylinder head, and forming a seal between the engine and valve cover.Valve cover gaskets are designed to be highly durable and long-lasting. However, they are also subject to wear and tear over time. Valve cover gaskets can become damaged due to excessive heat, improper installation, or age. Typically, there isn't a set replacement interval for valve cover gaskets — you don't usually have to replace them unless they're leaking or otherwise damaged. However, if your valve cover gasket is leaking, you should replace it as soon as possible. The most common symptoms of a leaking gasket include damp, greasy spots around the upper engine, oil dripping down the side or back of the motor, and puddles of oil on the ground. A minor valve cover gasket leak will likely only result in a dirty engine bay, while a severe leak can cause catastrophic oil loss.If you need to replace your valve cover gasket, don't worry. It's a relatively simple and straightforward procedure. A mechanic should be able to do the replacement in about an hour, depending on the car. But if you choose to replace the gasket yourself, remember to properly torque your valve cover bolts and follow the tightening pattern provided by your vehicle's manufacturer. Doing so can prevent future valve cover gasket problems.Improperly installed oil filterJust like your oil drain plug, it's also vital that you install the oil filter correctly when performing an oil change. Most oil filters have torque specifications. However, you really can't use a torque wrench on them due to their size. Instead, there are a few steps to follow when installing an oil filter to prevent leaks and other damage. First, you need to apply a light layer of oil to the filter's gasket. Doing so prevents the gasket from tearing during installation and also helps keep the filter from seizing to its housing, making it easier to remove during your next oil change. Second, when you install the filter, screw it on until it feels snug, and then turn it one-quarter to one-half turn more. After that, stop. Any tighter, and you risk damaging the filter or its housing.If you install your oil filter too loosely, you're likely to develop leaks around the housing. Like drain pan damage, oil filter leaks typically cause puddles of oil beneath the car. If you have an oil leak due to an improperly installed oil filter, the fix is simple — as long as your engine hasn't been without adequate oil for too long. All you'll need to do is replace the oil filter with a new one of the correct type and install it properly. Then, top off any oil you lost due to the leak. That said, if your car has been losing oil for an extended period, your engine may have developed more extensive damage. Any time you experience an oil leak, it's a good idea to visit a mechanic for an inspection and to determine the true extent of any damage.