* 5W-20: This is a common viscosity for many 2004 Explorer engines.
* 5W-30: This might also be recommended, especially in hotter climates or if your owner's manual specifies it.
Crucially, you also need to check the owner's manual for the oil type: It will specify whether it should be conventional, synthetic blend, or full synthetic. The manual will tell you the correct oil weight and type for optimal engine performance and longevity.
Never rely solely on online information for this. Consult your owner's manual to be absolutely certain you are using the correct oil. Using the wrong oil can seriously damage your engine.