My 18 required a regular 17 mm socket to remove. I now have a whole set of allen key bits that I guess I'll have on hand for Ikea furniture, or Kohler engine flywheels
As I said, it is a flange head, like the small middle picture comparing the plug types, so there is plenty of face for the washer. It's an outie, not an innie. The flange of the drain plug is wider than a new Mazda aluminum washer, but the washer I removed from the car after the dealer change had apparently been "crushed" into a much wider OD, smaller ID and was consequently thinner and difficult to remove from the drain plug/bolt.
It also seems that the dealer omitted one of the two plastic push clip rivet fastener retainers.
I didn't request the free oil change, the dealer does it for everyone, and I thought it would be good to get at least one multi-point inspection or whatever they call it, in case there was something unusual that jumped out at them. I'm not familiar with Mazda's, this being my first. Also it was a chance to get a feel for the service department. Generating a small amount of goodwill can't hurt either.
Otherwise, I prefer to do my own maintenance and part swapping repairs.
Asked the dealer what oil they used. It was a big name, maybe Mobile 1. Not Castrol or Mazda Moly. And I assume I imagined it, but seemed my mileage got a little worse after the 1st oil change.