I bought a car that have transmission problems.

I bought a car that have transmission problems it sometimes doesn't go to gear the old
owner told me they took it to midas they told them they need new trasmission. I just
took out the old fluid and it was dirty and I put new filter. I am just wondering, I been told
that I should use lucas stop slip with new fluid, other rumors say just new filter and ford
brand transmission fluid will solve the problem I need your advice, shall I put lucas stop
slip or just straight ford fluid? I have heard lucas can cause more damage. I need advice
thanks