The engine for the 6s (stock) is designed to run off 83 octane (regular unleaded), higher octanes, as i understand it, will actually be less eficient as they have a higher flash point. Now, if you race it and have modded the engine, the recomended octane rating will change. Hope this helps.
I use 83 and only from Sunoco, i dont use Sheetz fule, if you are familiar with sheetz u know what i mean. If i dont use Sunoco i find a Exxon.And NEVER fill up when the truck is there filling the gas wells.
When they fill the tanks at the station it stirs up a ton of sediment in the tank which you will then pump into your car. Not so much a problem if you've got a fresh fuel filter in your car, but it will definitely accelerate the filter clogging up.
87 Octane is the recommended fuel for Mazda6/3 except for the Speed models which require Premium. Running higher octane than specified in the manual only wastes your money as the fuel system mapping is calibrated for 87, and the car will deliver the fuel/spark in the air/fuel ratios as determined by 87 Octane.
If I remember right, it's because it stirrs up the contents of the tank and can lead to getting bad gas or crud in your tank. Now, don't quote me on that, but I seem to remember that being the reason I heard given for never filling up when the truck is filling up the wells. Wish I could give a more defenite answer.