Whenever in doubt I would just follow the manufacturers recommended intervals. They obviously know what their doing.
What do you mean by 4 and 6 here?My rule, since I always take advantage of Mazdas fun factor, is 3000 for a 4, 5000 for a 6. However, since the 2 uses a 0 weight synthetic, I do ours every 5000. Easy to remember, and is a pretty sure figure regardless of your driving habits.
i think you should change up your mobil oil as described by your car manufacturers.