Why does my gas mileage DECREASE after an oil change?

It seems like every time I change the oil in my car, I get WORSE mileage than before the change. What can cause this, and how can I fix it?
Update: I'm measuring my mileage per gallon by dividing the number of gallons to fill the tank (at automatic shut-off) by the number of miles driven on the previous fill-up. I use the same octane fuel every time. It has happened every time since buying the car (new) that within the first 2000 miles after the oil... show more I'm measuring my mileage per gallon by dividing the number of gallons to fill the tank (at automatic shut-off) by the number of miles driven on the previous fill-up. I use the same octane fuel every time. It has happened every time since buying the car (new) that within the first 2000 miles after the oil change mileage is between 28 and 30 mpg, and between 2000 and 5000 mpg increases to 30 to 33 mpg.
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