FORD 1998 Explorer - Manual (Page 163)

Engine oil recommendations
Look for this certification mark.
Ford oil specification is WSS-M2C153-G.
Use SAE 5W-30 motor oil certified for gasoline engines by the American
Petroleum Institute.
Do not use supplemental engine oil additives, oil treatments or engine
treatments. They are unnecessary and could, under certain conditions,
lead to engine damage which is not covered by your warranty.
Changing the engine oil and filter
Change your engine oil and filter according to the following mileage and
time requirements, whichever occurs first:
• Normal Schedule – 8,000 km (5,000 miles) or six months.
• Severe Duty Schedule - 5,000 km (3,000 miles) or three months.
Severe duty operation would include extensive idling, trailer towing,
driving in severe dust and police, taxi or delivery service.
Ford production and aftermarket (Motorcraft) oil filters are designed for
added engine protection and long life. If a replacement oil filter is used
that does not meet Ford material and design specifications, startup
engine noises or knock may be experienced.
It is recommended you use the appropriate Motorcraft oil filter (or
another brand meeting Ford specifications) for your engine application.
Maintenance and care
163