I have an 03 with a 6 speed.  Like others, I've had problems with the gears
grinding when shifting, particularly into 2nd and 3rd gears.  I tried Red
Line gear oil and it was worse, so I went back to regular gear lube.  Over
several months, it got to be so annoying that I finally took it to the
dealer, expecting to need a new transmission.  Their mechanic, who related
that he owns 2 Miatas (autocrossing one) said that they had many complaints
like mine since the 6 speed came out and that, in his experience, the 6
speed would never shift as smoothly as the 5 speed.  His disapproval, though
unspoken, was obvious.  After a test drive, he recommended draining the dino
oil and putting in ATF!  After being reassured about durability and the
warranty, I said OK.

What a difference!  It is like having a new transmission.  Each gear
"snicks" into place without having to pause in neutral to avoid gear
clashing.  2nd gear is still a little "nothcier" than the others, but I'm
back to having fun with my car.  I'm still hesitant about durability and am
keeping the invoice, but overall, I am pleased.

Has anyone else heard of anything like this?  Is this the equivalent of
using Red Line MTL (vs gear lube)?

Randy