Check your owner's manual: This is the most important source of information. The manual will specify the recommended viscosity grades for your Expedition's engine based on temperature.
Generally, for a 2002 Expedition in moderate climates, a 5W-30 or 10W-30 might be recommended. 10W-40 is often suitable for hotter climates or for engines that are showing some wear and could benefit from a slightly thicker oil at operating temperature. Using an oil that's too thick can put unnecessary strain on the engine, and using oil that's too thin could lead to inadequate lubrication.
In short: While 10W-40 *might* be acceptable, you should always prioritize the oil viscosity recommended in your owner's manual. Using the wrong oil can negatively impact your engine's performance and longevity.
