Mazda have always specified NGK BKR5E-11, BKR6E-11, BKR7E-11 or Nippon Denso K16PR-U11, K20PR-U11, K22PR-U11 plugs for the MX-5 - nothing has changed (and neither has the design of those particular plugs).

I asked a chap on the NGK stand at the Autosport show why NGK plugs were the best ones to use. I got the expected waffle - quality, tolerances, ignitability, etc. etc. Fact is - NGK only make spark plugs - they do nothing else (except leads, and I don't think they actually make them) and their products are being adopted as standard fitment by more and more car manufacturers.

Probably there isn't a lot to choose between the leading brands of plugs - it's just that from my (and several other people's experience) NGK plugs work more consistently than others. I've fallen for several gimmick plugs in the past - Golden Lodge, Splitfire, Bosch Platinum, etc (although not for many years now). None of them were worth a candle.

I've also had plugs foul, and individual plugs fail after a short distance (or even be faulty when first put in the car). I have never, ever, had a problem of any sort with an NGK plug, in any engine - including cars, motorcycles, scooters, lawnmowers, strimmers and outboards.

BTW - I use NGK ZFR6F-11 in my car (1/03)