Exactly the weight that is listed in the owner's manual and possibly on the oil fill cap. any Major Brand is your choice. Or you could call a Ford dealer.
What type of oil does a 1997 Ford Mustang 3.8 V6 use?
Use 10-30
10w30
Petroleum oil
don't no
5w-20
the black oily kind
75w-140
( 5W-30 ) is the preferred oil according to the 1997 Ford Explorer Owner Guide
The manufacturer recommends 10W-30 weight oil for the 2005 Ford Falcon. A heavier oil can be used in extreme climates.
I use 10W-30 Pennzoil. Use 5W in the Winter.
Use the oil of your choice. 5w30 for colder climates, and 10w30 for warmer weather.