It is possible to mix 5w30 and 5w20 motor oil in your car, but it's not usually recommended. Many carmakers recommend using the same weight oil year-round, so if you switch to a lighter weight during the summer, your engine may not be protected against wear and tear as well as it would be with a heavier weight oil. Additionally, if you mix two different weights of oil, you may not be getting the best possible protection for your engine.