The kind of oil the 2015 kia optima takes is5w-20 or 5w-30. your oil needs to be changed eventually. every 5,000 to 10,000 miles is a good rule of thumb. if you don't change it, there will be problems with the engine. having high quality oil is essential for the engine to run smoothly. you will run into issues with this fluid at some point. a normal problem is a nasty burning smell. this needs to be looked into as soon as possible or the car might explode. bad oil can cause the engine to fail. you could experience bad gas mileage. knocking noise may show up from the engine.
Synthetic or conventionaloil
The recommendation for all 2015 kia optima models, except the turbo charged models, take a 5w-20 or 5w-30 viscosity engine oil. the turbo models take a 5w-30 or 5w-40 viscosity and oil that meets acea a5/b5 quality standards.