I've posted a lot on this, but, best we "know" is
Info on it is not definitive, and not official, but enough people have
encountered problems (sometimes serious, requiring head replacement)
that questions remain... (other people report no problems, though).
Not long ago, Sony changed its tapes to type-II, and rumor had it that this
was to improve compatibility with other brands (leaving the question of
whether or not mixing old and new Sony tape is a problem [it seems to me
that I did have more dropouts for a while when using both new and old tapes...]).
Basically, the manufacturers say nothing; some people report mixing tapes often
is OK (but most of the brands are made by only two makers, so it is hard to
know when you are really "mixing" tapes), some report problems with long-term
use of one tape then switching; some report problems with ANY mixing; some
report that mixing is OK so long as one avoids using entirely the high-end
Panasonic tape with the white case and blue cover (or uses it exclusively...); in
other words, to be safe, select a tape, and NEVER CHANGE, EVEN ONCE.
People do report fewer problems with Sony than with Panasonic, though...