Once you know the engine size, look in your owner's manual for the recommended oil viscosity (like 5W-30 or 10W-30) and API certification (like SL or SM). The manual will specify the correct oil type for your climate and driving conditions.
Generally, for older vehicles like a 2000 Explorer, a conventional oil meeting the manufacturer's specifications is sufficient. However, you could also use a high-mileage oil formula if your vehicle has high mileage. Again, consult your owner's manual for the most accurate information. Ignoring the recommendations in the owner's manual can damage your engine.
