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 safest, select a tape brand, and NEVER CHANGE,
EVEN ONCE.
(But there really is nothing *really* definitive out
there to go on, really...;-)