[ben]

The problem is, there could have been damp on the tape surface which has evaporated leaving behind who-knows-what. There could well have been a chemical reaction with the particles, binder, etc. Maybe not visible to the naked eye but could harm the machine as Nick says.

Perhaps you could try running them through on a junk machine first.

I would bin any suspicious looking tapes, or any giving poor results. Just not worth it! I recently found a stash of mid 80s beta tapes, you would be welcome to a few FOC but unfortunately, BC is a bit far from me!