Why do you even need oil? Cars need oil to help keep the fast-moving parts in the engine well lubricated. If you are low on oil, and consequently have friction in those parts, it can be very bad, and very expensive.
If you use the wrong oil in your car, it won’t be an immediate disaster like it would be if you put the wrong fuel in, but a few thousand miles down the line, you will have some big issues, and it will decrease the life of your engine.
To find out which oil you need, all new cars come with a recommended grade – and likely a recommended manufacturer – listed in the manual. It will be shown in numbers, separated by a w, such as 5w30.