The recommended motor oil for a 2003 Hyundai Sonata is a semi synthetic 10w40. Some name brands that manufacture this type of oil include Castrol, Lucas, Valvoline and Liqui Moly.
A 2003 Hyundai Sonata must have its oil serviced every 3,000 miles. Oil changes should be scheduled according to the car's driving history and the severity of driving conditions. Severe driving conditions that affect the frequency of a Sonata's oil maintenance include repeated short distance driving, excessive idling, driving on sandy or dusty roads, driving in heavy city traffic during hot weather, and driving in cold weather conditions where salt and other corrosive materials are on the road.