The O/D light in the dash started blinking today in my Grand Marquis. What causes the light to blink like that?

Also the tranny isn't in such great shape over all. It slips a little in first gear, but has been slippping for like 3 years already. I'm suprised it still works. It was pretty much dead one morning. The car wouldn't move but then it came back to life. I was told that the Lucas tranny fliud will help. Has anybody used it and does it actually work? I would like to squeeze a few more months out of the tranny if possible.

TIA