I use nothing but old tires on my Fluid 2 for the last 13 years and no problems, it's a waste of money to go by a special tire. HOWEVER, if you use an old tire you have to inspect the tire real closely and pick out any objects imbedded in the tire, then you have wash the tire with a scrub brush to make sure all the dirt is off. Any dirt or embedded objects can ruin a drum on a trainer.
The big problem with people thinking they have to use a special tire is the problem with regular tires shredding bits of tire everywhere and a trainer tire won't do that, and this is what you're reading on the internet as to why it's best to use a special trainer tire. But the issue isn't with the tire, the issue with tires shredding is the fact people don't have their trainer drum adjusted correctly against the tire! They put too much pressure against the tire and go off riding it only to see tire bits everywhere, so a couple of tire companies recognized that people were having this issue and they could make money selling a tire that wouldn't shred.