What kind of motor oil do a 1996 Seville use?

There are a few variables you need to take into account. Most importantly is the temperature that the car will be operated in. If its a colder climate stick to a 5w-30, warmer maybe a 10x-30. If the car burns some oil stick with the 10w-30 otherwise 5w-30 is perfect for you. Synthetic will prolong your oil change intervals but if the car burns oil it will become expensive quickly! Synthetic oil paired with a synthetic oil filter can give you a 5000-6000 mile oil change interval compared to that of 3500 or so with "regular" oil.