5 w 30
The best oil is what is recommended by Ford - check your owner's manual.
5 W 20
My ( 1991 ) 5.0 LX Ford Mustang always used 5W-30 as recommended
( 5W-20 ) for the 4.6 liter V8 in a 2007 Ford Mustang
( 5W-30 ) according to the 1999 Ford Mustang Owner Guide
The 1999 Ford Mustang 3.8 liter V6 engine takes : ( 5 quarts with engine oil filter change )
5 quarts ( 4.7 liters ) WITH FILTER CHANGE - according to the 1999 Ford Mustang owners manual
Try above oil filter
5W-30 , according to the Owner Guide
Varies by climate, but 10-40 is pretty average
According to the 1999 Ford Mustang Owner Guide : With engine oil filter change ( 5.0 quarts / 4.7 liters of engine oil )