Other factors are at work here, not the oil. And as far as the A/C goes, it is either on or off, meaning that no matter the setting, the compressor is going to be either on or off. When on, it engages and disengages at relatively the same rate no matter the temp or fan speed setting. And yes, using the A/C will slightly decrease mileage because of the extra load on the engine. My best guess is it has more to do with the increasing summer temps outside.
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