The 2015 kia optima comes in five different configurations, across which two different engines are used. you'll either have a 2.0l or a 2.4l 4cylinder, each of which uses5.1 quarts of oil. the 2.0l requires the use of sae 5w-30 full synthetic motor oil, though sae 5w-40 conventional is permissible in the short term. the 2.4l requires sae 5w-20 full synthetic, though sae 5w-30 is permissible. however, in both cases, the first oil listed is strongly recommended. the reason for this is because of superior viscosities -- lower numbers will, by and large, keep your engine running more efficiently, which can combat friction and premature wear and tear. this will lead to your engine lasting longer, so the few extra dollars is worth a few extra years with your optima.
The 2015 kia optima takes about 5.1 quarts of engine oil.