Hearing about some clutch problems from worn-out brake fluid on cars (not just the mustang), the other day, I Mity-Vac'd the fluid from the master cylinder and it was pretty yellow and discolored after 5000 miles. I think the clutch slave cylinder gets pretty hot and helps cook the fluid. The clutch did seem to engage a bit higher off the floor after the change.
My usual routine for brakes is to use the Mity-Vac at the calipers and vacuum until fresh brake fluid appears. On cars I do it every two years, on my bikes, every year. Since the Mustang's fluid seemed pretty gunky, I'm going to do the master cyclinder every 5000 miles and the calipers every two years. Cheap insurance.