at my last oil change and looks like my MPG is not what it used to be.. On my regular work commute I'd hit 19mpg without really trying but now I'm at low 17 the past few days since the oil change..

Anyone else experience this? Does it take a little while for the car to get used to the new oil?

0/40....5/40......makes NO difference......IMHO of course.
Cheers, Pickles.

Cant believe it would make much if any difference to fuel consumption, certainly not to the extent that the OBC could detect. Do you think your right foot could have become a little heavier, checking out whether the new oil has improved performance?

While it may be possible that your MPG has changed...I am wondering why someone with a high performance car with higher performance upgrades (ie EC tune, and BMC's) would be concerned with MPG's? There is a reason this car comes with a Guzzler tax and it is not because it is a conservative drinker!

could be, but 80% of my work commute is cruise control, too many cops around, i've learned my lesson plenty of times.

i daily drive my car (including 40mi each way for work) and although I've learned to not really pay attention to how much i spend on gas in a month, i do try to keep track of how much mpg i can get on the highway.

Many OEMs now specify a multigrade oil as low as 5W-20 to improve fuel consumption once the car reaches operating temperature. High revving performance engines need a higher viscosity at the top end to protect from heat and improve the flash point.

The fact that you switched to 5W would make little difference in my opinion but the oil may be a little bit thicker.

coincidence

Have you checked the level?

Maybe oil level too high.

Both are 40W at running temp and would be the same viscosity.

Only at 0c are they different weights.

The first number is viscosity rating and the second number is the flash point. The difference between 0 and 5 weight is minor but manufacturers use 0-40w as it is thinner at low temps and heats up faster for better emmisions. As for engine life you're better off with 5-50w unless you live someplace very cold and you park outside.

^both numbers are viscosity ratings, not flash point(which would be in degrees C)
first number is when the engine is cold and second at operating temp.

That's an incorrect assumption. Although they are both 40W spec their viscosity can be different at higher temperatures. Generally the further the distance between the "Cold" and "Hot" viscosity rating the more synthetic material in the oil.

The 0W40 is actually a little thicker at @ 100º C because it has a higher viscosity index.

0W40 Spec Sheet is 13.5 cSt @ 100º C

5W40 Spec Sheet is 13.2 cSt @ 100º C

Wowzers a 2% difference, thanks for clarifying that.

A 2% difference can change your life