ok, I know old tranny fluid is not good for your auto but just what does it do to it? like lately I've been driving back and forth on the interstate. I drive it about 35 miles to knoxville, do a few things there, then drive back home. right before I get home, the check engine light come on for like 1 minute. after getting off the interste and drive through town to go home, the tranny sorda goes throughthese fits where it shudders and makes a slip sound. my stang has about 75k miles on it. fluid seems to be ok but sorda hard to tell. does this sound like bad fluid or has my tranny finnally bit the dust?