My mileage dropped as well after my last oil change. My service order ticket stated that 5W30 was used. I have brought this to the attention of the service manager and he claims that 5W20 was used but the computer software that prints the service tickets was wrong and assured me the correct oil was used. That being said, I have noticed a decline in the MPG since then but also even more so since it got cold. 24 degrees this morning and todays avg is 34.5 instead of my normal 36.5 or so. I believe as other posters has stated, that cold weather will effect your mileage. I have to believe the service manager who told me that they have used the correct oil. I wish there were a better way for them to prove that to their customers. If, in fact 5W30 is used, what are the problems?. I know the recommended oil is 5W20 and there is good reason for that to maximize economy, but what is the downside of oil that is a little thicker for adhesion?
Just curious.