My 1991 Mustang 5.0 LX came from the factory with ( 5W-30 ) I ASSUME your Ford Thunderbird would be the same
Ford did not make a festiva for sale in the us in 1997. They made and sold an Aspire auto trans or straight drive
what type transmission fluid uses for a 1991 ford probe
www.everdrive.com will tell you
5w30
Ellipse.
10w30
5.0 v8 made by ford motor co.
look on sticker on door tells u what type
10w30 or 10w40
Family owned, & public company.
It really depends on how fast you need the motor, and what type it is. I would suggest going to your local ford house and asking for an estimate.