However, generally speaking, a 1998 Ford Expedition likely used:
* 5W-30: This was a very common viscosity for that era and likely the most frequently recommended.
Crucially: The owner's manual will specify the *type* of oil (conventional, synthetic blend, or full synthetic) as well as the viscosity. Don't just rely on the viscosity; using the wrong type of oil can damage your engine. Consult your owner's manual or a reputable auto parts store (giving them the engine size will help).