2004 Mazda 3 Recently Noticed That Getting About 1 Mpg Less Mixed Driving

There does not seem to be a trust issue here, as far as with your chosen mechanic. I dont perceive any wrongdoing, and there may be factors not included in the words in your post. Most car manufacturers use adaptive systems and it may simply be a matter of allowing the car to adapt. 2 mpg difference by itself is not alarming. I would look at the additive and multiplicative fuel trim and see if it is at or near its limit. There are many factors to consider, lets take them one at a time, agreed?

I don't see your location but I suspect the weather is cooling off which will reduce the fuel mileage. Oxygen sensors get clogged and becomes less sensitive and accurate which will also effect your mileage.

I disagree. I think 5 mpg on the highway is significant. That said, I have a previous mechanic, and he supported your suggestion that their might be issues with my fuel. I disagreed until, my buddy suggested that different formulations with the cooler weather can create problems. I took his advice, and ran a can of Seafoam on a quarter tank of gas, and then, I've been running premium fuel from Exxon. I appear to be back to my previous mileage. As for the O2 sensor, I asked them that, but they said it checked out fine. Thanks, for the input.