Hi , my name is Casey and I just bout a 2000 mustang convertible with the 3.8 auto trans . Found a trans fluid leak while making repairs that it needs . The trans shifted like a champ in every gear when I bought it about a month ago . I’m guessing developed a leak and needed to add fluid . this is the problem ….. I was told to get trans fluid for automatic ford transmission. I had no idea it needed merc V in this car . So with the wrong fluid in it it started not downshifting like it should immediately but still goes into drive and reverse perfectly when you crank up for the first time so . I thought it might be leaking bad and still low due to the large puddle and added another quart of the wrong fluid .when it didn’t act right the 2nd test drive (about a mile trip ) I parked the car and haven’t cranked it since hoping it could be fixed without a new trans . My question is should I do a flush and change filter or just drop the pan and change 4 quarts and filter with the merc V . To my knowledge it has never been changed before . 200000 miles roughly on it . Any advice on the best thing to do next would be appreciated .