2006 Scion xA - Worse mileage after oil change

 First off Ray, I have enjoyed your column ever since Click & Clack.  I look forward to reading your column every Saturday in Virginia Beach, VA.     
 My son owns a 2006 Scion XA with 206,000 miles.  I change the oil & filter for him every 5,000 miles or near about and do most basic repairs when needed.
 He tells me that he gets worse gas mileage right after any oil & filter change and better gas mileage right before the oil & filter are changed.
 Can this be possible?  My only answer to him is that maybe the oil is thinner and has lost some viscosity than the new oil?  Or maybe he drives more on the open highway at that time before oil changes?
 I am stumped and would like to know myself if this is possible.

First Robert is no fun to read a post that needs scrolling . All I got out of this that your son gets worse fuel mileage after an oil change . Then tell him to do it his self . Frankly I wonder if he really is checking the MPG correctly because that oil change will not make a difference.

Agree with @VOLVO_V70, it should make no difference. I thinks it is all in his head.

You might ask what kind of oil they are putting in and if it is what the book calls for.

Ray (Oh, nevermind, Ray doesn’t come here)

An oil change will make no noticeable difference in fuel mileage all things being equal as far as driving habits go.

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Unless you are keeping the car idling for half an hour for the oil change, then it shouldn’t matter.

Are you over filling the crankcase? I have had customers complain about rough idle after someone over filled the engine but not a change in fuel economy, over filling might have an affect on fuel economy.