My gas mileage worsened after oil change on merc villager?

I recently had an oil change at wal-mart. The week before i had it changed i check my mpg and was getting 23 mpg average. Now I'm getting 17. My check engine light came on, the day after I had the oil changed, I called w-m and was told to bring it back. By the time I drove to the store the light was off and now... show more I recently had an oil change at wal-mart. The week before i had it changed i check my mpg and was getting 23 mpg average. Now I'm getting 17. My check engine light came on, the day after I had the oil changed, I called w-m and was told to bring it back. By the time I drove to the store the light was off and now I'm getting bad gas mileage.
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