I changed my oil about 1000 miles ago, and used Mobil regular (Drive Clean 5000) oil, 10w30* as the manual says.
I've noticed a drop of about 1-2 mpg since then, and my driving conditions have not changed significantly. Looking at the graph of my mileage data it is pretty clear--it dropped immediately after the oil change, by about 1.2 mpg.
Is the factory fill a 10w30 conventional oil? Or is it something else (partial synthetic, lighter weight, etc.)?
*Edited to add: I had my viscosities mixed up. Manual calls for, and I put in, 5w30.