FORD 2006 Explorer v.2 - Manual (Page 290)

To avoid possible oil loss, DO NOT operate the vehicle with the
engine oil level indicator and/or the engine oil filler cap removed.
Engine Oil Recommendations
4.0L Engine
Look for this certification
trademark.
Use SAE 5W-30 engine oil.
Only use oils “Certified For Gasoline Engines” by the American
Petroleum Institute (API). An oil with this trademark symbol conforms
to the current engine and emission system protection standards and fuel
economy requirements of the International Lubricant Standardization and
Approval Committee (ILSAC), comprised of U.S. and Japanese
automobile manufacturers.
To protect your engine’s warranty use Motorcraft SAE 5W-30 or an
equivalent 5W-30 oil meeting Ford specification WSS-M2C929-A.
Do not use supplemental engine oil additives, cleaners or other engine
treatments. They are unnecessary and could lead to engine damage that
is not covered by Ford warranty.
Change your engine oil according to the appropriate schedule listed in
the scheduled maintenance information.
2006 Explorer (exp)
Owners Guide (post-2002-fmt)
USA (fus)
Maintenance and Specifications
290