Checking your oil is one of the easiest things you can do on your vehicle. It is important to regularly check your oil level and oil filter.
When the oil change is done, all old motor oil is drained completely and it is replaced with the correct oil suggested for your vehicle. (your car manual should tell you which oil and how much)
There is some controversy on when a vehicle needs an oil change and new filter but it is best to stick with what the manufacturer suggested duration for your car.