The recommended oil grade for a 1999 Ford Explorer with a V8 engine depends on the climate where you live. Ford's owner's manual (which is the best source for this information) will specify a range of acceptable viscosities. However, you'll commonly see recommendations around
5W-30 or possibly
10W-30.
Always consult your owner's manual for the exact specifications for your vehicle. Using the wrong oil weight can damage your engine.