In my last oil change I ran Valvoline Advanced 5W30 (no issues) before changing oil out to QSUD 5W20 ... Surprisingly , for the first couple of days the engine ran slightly rough and the gas mileage actually dropped before smoothing out and the gas mileage improved . Is there any truth to the belief that this happened as a result of the new oil needing a little time to remove the old add pack on metal surfaces before laying down it's own new add pack on metal surfaces and smoothing out ?