According to the 1997 Mercury Mountaineer owners manual : Your 4R70W automatic transmission takes ( Motorcraft MERCON automatic transmission fluid ) * the type that is used should be engraved into your automatic transmission fluid level dipstick *
For a Mercury Mountaineer : ( " regular " unleaded gasoline , 87 octane )
4F20E four speed automatic.
What kind of car 1999 mercury mountaineer
According to the 2002 Mercury Mountaineer Owner Guide : For the 4.0 liter SOHC , V6 engine ( 5W-30 ) For the 4.6 liter V8 engine ( 5W-20 )
RE4F04A, RE4F04B or RE4F20E, depending on your model year. They are all the same basic unit, a 4 speed automatic.
You have to give us a little more info than that for us to give you any kind of answer.
That would be a 4L60E transmission.