The recommend motor oil weight for a particular car is listed in the owner manual that came with the car when it was new, according to Cars.com. The oil cap under the hood may also have a sticker printed with the recommended oil weight.
The owner’s manual may also recommend different motor oil viscosities for different conditions. Common viscosities for motor oil, as of 2015, include 5W-30, 10W-30 and 5W-40. Many automakers require that car owners use the recommended oil or risk voiding their warranties, since using a heavier-weight oil than recommended or using synthetic oil that is not recommended for the car can cause engine damage.