Hi Richard,
I used a Cretacolor 6B graphite stick. I held the stick flat against the binder
side of the tape as it played, at a spot just after it left the playback reel
and before it reached the first tape guide. As I recall, it may have taken two
passes to eliminate the squeal completely, but the results after the first pass
were encouraging enough to warrant a second try. I rotated the stick
occasionally because it seemed to get worn and shiny at the point of contact,
and to acquire a buildup of crud (probably from the tape).
I'm guessing the softer grades of graphite are better for lubrication. I understand it is also sold in powder form, but the stick seemed easier to manipulate.
Rick Taylor Austin, Texas