When you say motor oil, its not just one type, there are several types of motor oils. You cannot just any, if you do it will leave a bad effect on the performance of your car in long term. Or in some cases it might breakdown your car.
So you have to choose the right motor oil for your car. When you go to a shop to get one, always check first according to the make type of your car and then narrow it down.
Best way to match it, check your car manual and find the right one according to it. Or remember the type of your car and read the labels of the motor oils in the shop, which ones are suitable for your car.
If you do not know then ask a member of staff to help you find the right one.