tranny slips when cold

ok i have a 1986 buick regal limited with over 190,000 miles on it. now this car rides good with the exception of slipping when the engine or trans is cold. if the car sits up for 6-8 hours after starting the car and letting it run for ten minutes. when i attempt to drive the car it holds in first gear. i would get it up to 24mph it will hold there while the engine revs and the car feels light it is in neutral but it is not. but after driving like that for a minium of 1-2 miles the car drives like normal. if i change the fluid and filter will it make it worst or should just keep driving it until the tranny goes out. also will lucas trans fix work or some other type of tranny fluid that stops slipping.

You can try an additive but realistically at 180k and 20 years, the internal seals are done.