Agreed that behavior in gravity un-spooling is telling. Yet I know of
at least one case where layer-to-layer adhesion was not a binder
chemical degradation issue, but a mechanical one. The tape was out of
tolerance for width (slit too wide), had edge fraying resulting from
playback on the wrong equipment, and when wound and stored under
tension, the frayed edges stuck together, slightly.

Squealing can also stem from both chemical and mechanical causes.
Many, many tapes in fact squeal, it's just a matter as to what degree.
You can even make good tapes squeal audibly. Looking into this
phenomenon with acoustic detection is useful and educational. For me,
it's been fascinating.

You certainly do not need to order the ASFDS option mounted into one
of our expensive SHRO headblocks to start listening to tape mechanical
motion sounds. Remarkably inexpensive miniature microphone capsules
and audio op-amps are available. Just chose a pair of headphones and
make your own hand-held electronic acoustical stethoscope. Then you'll
have a tool that ought to be in the possession of anyone working on
professional tape machines.

ATL doesn't use tape lubricants. Perhaps because we don't have to.
When a tape's lowered glass transition temperature is causing
problems, why would you needlessly heat up the tape with unnecessary
friction in playback? As someone in this thread hinted at, low
friction is the goal. So get rid of the erase and record heads!

And stop believing the nonsense promulgated by old-school recording
industry people. You do not need to keep a tape machine record-capable
in order to be able to align and test it.

Instead, chose a reproducer tape transport that is servo constant
tension, has all-rolling straight-line bypass of the headblock in wind
modes and that allows easy, independent tension adjustments to be made
by the machine operator. And properly train your operators.

For those who are stuck using tape transports with fixed pin lifters,
try to always thread bypassing the headblock and lifters when
re-winding, if it is possible to do so safely. Viewing tapes suffering
from a softening of their binder chemistry, under stereo microscopy,
quickly convinces one that fixed pin lifters are to be avoided.

Fred Thal