[10fold]

I was driving yesterday and noticed that my third gear grinds whenever I downshift into it. I looked through the forums and read about replacing the synchros and was wondering if there is anything else I can try to fix it. It's strange because it's only when I downshift, from 2nd to 3rd is fine, but 4th to 3rd grinds. Maybe I need to change my tranny fluid? I can't even remember the last time I did it, I just got done with school for the semester. Thanks in advance.