1997 Lincoln Town Car Finding Out If I Have Slip Diff.

LSD, i would like to find out after changing my oil in my rear, i need to add some type of additive to my oil if i have rear slip diff. im not sure if i do. but i do have a stamp code that was on the casing and held on by the bolt. here are the numbers. top line = S040L second line = 308 88 6M04.
After removing the axle cover, i drained the oil, cleaned the housing then, cleaned the housing cover, applied my cilicone bead. i dont know though if the car is equipped with Traction-Lok differential. to add the friction modifier additive.