Yes it can decrease mileage. You have to take the sensor out and look at the two sensor resistors. If they're dull grayish in color, they're dirty. If they're shiny silver, they're clean.
I use electronics cleaner, and it made a HUGE difference the 1st time I did it.
It hadn't been cleaned the life of the car at that point though, 60k miles or so.
I do it every oil change, and every time I clean and re-oil my air filter.
I have used MAF cleaner, Brake cleaner, Starting fluid, carb cleaner, rubbing alcohol, electric parts cleaner, electric motor cleaner.... they all do a great job.
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