FORD 2003 Explorer v.4 - Manual (Page 235)

4.0L Engine
Look for this certification
trademark.
SAE 5W-30 engine oil is recommended.
Only use oils “Certified For Gasoline Engines” by the American
Petroleum Institute (API). Use Motorcraft or an equivalent oil meeting
Ford Specification WSS-M2C205–A.
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.
Change your engine oil according to the appropriate schedule listed in
the scheduled maintenance guide.
Engine Oil Filter Recommendation
Change your engine oil filter according to the appropriate schedule listed
in the scheduled maintenance guide. 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, start-up 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.
Maintenance and Specifications
235