The lack of knowing the correct fill procedure is or was the problem. Like I have told many owners on this forum, if you don't have the expertise to fix your car, you will be better off taking it to a tech and let them fix it rather then trying to save a couple of bucks and causing more damage to the X. Using the wrong fluid in the first place and having to buy the correct fluid and flush the wrong fluid probably cost you more then if you just brought it in.
here's a link to the Rover forum which also uses the same ZF trans.
5hp24 Transmission - Whining after Fluid Change
Most likely by driving with a low fluid level, you may have cause additional problems or wear on the trans. When you started having this problem 1-1/2 years ago you should have just brought it to a tech. They would have perform a fluid level check and just inform you of their findings.
Good Luck.