@milky managed to break the transmission yet? Ive heard ND transmissions are made out of glass.
Mine went at 11,000mi - not abused, not even a single AutoX, just commuting and highway driving. I was accelerating onto the interstate, shifted from second into third, heard crunches and rattling, and fourth gear was locked out. Second was locked out too. Got to 6th okay, called the dealer to let them know, dropped it in and picked up a loaner Miata, which I drove for a month while they awaited trans replacement to arrive.
My car was a Version 1 trans, then ended up with a Version 3 trans that gave no more issues or the subsequent 20k miles. There is still no verdict on the failure mode for the transmissions, though lots of suspicion. I believe it relates to poor metallurgy of the gears and gearshafts (they've been the center of Mazdas revisions) and possibly the case deflection itself, since most of the 'lightweighting' came from varying the casting thickness of the manual transmission.
I'd also note that I know people who have been way harder on their ND than I was and never had an issue. Seems to be a variance that doesn't affect all cars.
@mr-ontop not at all! The original owner ordered the sport package with the extra chassis bracing and larger brakes and LSD, but declined the option for abs. I don't think I'm the first guy to use it for autocrossing or track days.
After my first Miata oil change, I didn't tighten the oil filter down enough for the same reasons as you mention. A couple months later at my next change I found my "leak". At least it was an easy fix!