Quote Originally Posted by Spadesheart View Post
I've been changing the oil with my own stuff for a while as you can get the synthetic for fairly cheap in Canada. There was one time last fall where I got the oil changed with a conventional oil as I was well above the limit and just needed it changed. I don't imagine that would have too much of an adverse effect. At least I would hope.
Are you saying the car runs synthetic all the time but on one occasion you simply used conventional?
How that relates to mpg I'm not sure but unless you're running a synthetic "blend" oil, the change to conventional can have significant ill effect like clotting/gelling which can plug filters and passages. Could it be that you're still seeing these effects in your fuel economy?