First the moly, then filters, plugs...
Next I will be told that I must get my fuel from the dealer's SkyActiv pump exclusively!
I consider each separately. In the case of the air filter, it would be difficult to make the case there is one better than the OEM filter (and it's very reasonably priced). At least I've never seen a convincing argument there is something better available.
In the case of the motor oil, the Mazda oil tests very well in independent tests, and is not too expensive. Perhaps someone could make the case that you could save a couple bucks and have similar performance. But I like the idea of molybdenum coating the wear surfaces and no one has made a good case that another oil would be better.
In the case of the oil filter, the Mazda OEM filter has been cut open and the inspection determined it has quality construction. It's also very inexpensive. Some aftermarket filters cross reference to vehicles that Mazda recommends a different filter for. That troubles me as bypass pressures can be different. This is a no-brainer because the OEM filter is so inexpensive and you know it has the approved bypass pressure,
Now with spark plugs we have an entirely different situation (at least with regard to information that has been posted on this CX-5 forum). Primarily the fact that OEM plugs retail for about $100/set while the Denso plugs retail for $32/set. If I can replace my plugs with the functional equivalent for $70 less, guess what I'm going to do?
Now the gentleman from OVT has alluded that they are not the equivalent by saying "Without getting into too much detail..." Yet he provided NO detail as to why the Denso plugs are not the functional equivalent. Usually, car enthusiasts want to share their knowledge, not get into cat and mouse guessing games. I'm not saying the Denso plugs are the functional equivalent because I really don't know, they could have different resistance values, etc. But without specific evidence to the contrary, I'm going to spend $70 less. Then, if my MPG declines, I get radio interference or lose power/torque, it's a simple matter to switch back and I'm only out $32. But before my plugs are due for replacement, I expect there will be other data points provided by other owners (or perhaps OVT himself) that will fill in the blanks.
But no one on this forum has ever forced anyone to their belief, people have just stated their opinions and (sometimes) supported those opinions with facts or logical reasons supporting their opinions.