Adding only the XL18 to your old fluid will be a waste of money and time.
You will see far more improvement by changing the trans
fluid to QDC. Then the XL18 will improve on that a little.
I changed the fluid in my 12 GT after driving the car for nearly 2 years and it almost eliminated the 2 gear roughness, a month or so later I added the XL18 and I haven't had any 2 gear roughness since.