Is it important to use only one type of oil in a car?

Oil comes in different weights, generally 30 or 40 (but the others are 20, 30, 40, 50). Thinner weight oil has a lower weight number. Thin oils are used in cold climates or wintertime because temperature can make the oil congeal. The 40-weight oil, a thicker oil, is used in warmer climates or summertime so the heat will not thin it out too much and not protect the engine.

If you mix one weight of oil with another weight of oil you are defeating the principle related to how heat or cold affects oil. Mixing two weights can cause the total oil to become too thin or too thick. Either way your engine could have problems.

The simple reminder is, if it is cold outside lessen the weight. If it's hot outside, increase the weight.