Shifting Rough

Hi, I have a 1990 Chev Lumina Euro with 145k miles i just got this car for free from my Aunt and i am very happy with it so far except it has shifting problems. When i took it to valvoline to get the fluids and filters changed,they told me they couldent change the trans fluid because it was a dark brown color. It might damage it more than it already is. When im on the road the car doesnt "slip" it shifts very rough like its slaming into gear. Same thing when i go from park to drive or overdrive. Is this a common problem or should i be looking to get it fixed right away?

With that kinda mileage, and the burnt fluid, it's a pretty good bet you'll be sinking some money into that tranny soon. If it is acceptable to drive currently, I'd go ahead and let the fluid in the pan go, new filter and refill with new fluid and a quart of Trans-X, you might get lucky. Otherwise, start looking for the best price on a rebuild.

what is trans-x, and what does it do?