Your car's oil works to lubricate the moving parts of the engine, ensuring that you get the most efficiency from your engine. Over time, dirt and grime can build up within your car's oil, which can gum up the engine and hamper the proper operation of your vehicle. Understanding what to look out for can help you more easily determine when it's time to take your car in for an oil change.
Open up the hood and take the oil dipstick out for inspection. Ideally, your oil should be a semitransparent yellow-brown color, but over time as it becomes dirtier and will begin to turn an opaque black. The darker the oil is, the dirtier it is and the sooner you should head to a mechanic. Leaving this unchecked can cause a number of problems in your car: the grime in the oil can gum up your engine, which means that the performance, fuel efficiency, and responsiveness of your vehicle will all suffer.
If you're still unsure of the state of your oil after checking the color, try rubbing some between your fingers. If it's clean, the oil should be slippery (thus giving your engine adequate lubrication). However, if you feel any sort of solid material in the oil, or if it does not feel slippery, it's probably a good idea to have your oil changed as soon as possible.
Checking your oil is not something you usually do every time you operate your vehicle. Another sign that you need to change your oil is an increase in the noise of your engine while driving. If it sounds rougher or louder or emits any sort of grinding sound, you should head to a mechanic. Improper lubrication can cause increased wear on your engine's pistons, which can be expensive to repair if they end up getting damaged.
If you've added oil to your vehicle recently but still have a low oil level light coming on in your car (or find that the oil has dipped below the minimum indicator on the dipstick), there may be an issue. Either there is a leak somewhere in your vehicle in which your motor oil is leaking out of your car, or your engine is gummed up to the point where it requires a great deal of lubrication in order to continue operating as it normally should, both of which need to be fixed as soon as possible.