In your owners manual , page 302 , comment # 5 shows that it is NOT MANDATORY to use synthetic or synthetic blend motor oil in your engine as long as the motor oil meets Ford specifications and the API " certified for gasoline engines "
Your 1991 Ford Explorer 4.0 liter takes 5 quarts ( 4.7 liters ) of motor oil WITH FILTER CHANGE
10w30
5.0 liter motor
The only motor it came with from the factory is the 4.0 liter - OHV - V6
The 4 liter Ford engine is a V6 motor,meaning there are 6 cylinders,3 on each side of the motor.
The smallest engine in a 1999 Ford Explorer is the 4.0 liter V6
For a 2003 Ford Explorer : There were two engine sizes offered from the factory ( the 4.0 liter SOHC , V6 engine , and the 4.6 liter SOHC , V8 engine )
There were ( 2 ) different ( 4.0 liter V6 engines ) The 4.0 liter EFI ( the 8th " character " of the VIN is an " X " ) and the 4.0 liter SOHC ( VIN " E " )
Ford uses the Motorcraft blended oil (part synthetic).
The recommended engine oil for both 1.8 Liter turbo and 2.8 Liter VR6 is 5w30. I personally recommend 5w40 synthetic on the turbocharged 1.8 Liter.
According to the 2005 Ford Explorer Owner Guide : With engine oil filter change : The 4.6 liter V8 engine takes ( 5.7 liters / 6.0 U.S. quarts of 5W-20 )
The Ford 4.0 liter SOHC - V6 engine has ( 3 ) timing chains