I recently changed my oil from using 5w-20 (penzoil) to 5w-30w(redline) and I noticed decreased in mileage. I use to be able to go 320 mi before the gas light comes on but now gas light comes on at 280 mi.

Is there that much difference in in mileage just by change the weight of the oil?

 

Yes, my dealer put in 5w 30 oil and I went from 19mpg around town to 16mpg and from 29mpg on the road to 26mpg.

Once I had him change it back to 5w20 the better milage returned.

Do search on oil for many many more posts.

 

I went from the break in oil directly to Mobil1 5w-30 at 4200 miles and my mpg's were not affected. I am still getting 21-22 in the city and 29-34 on the highway.

This is not to say that switching over to 5w-20 now would not increase the mileage although it is pretty good. Also, remember, my driving style is usually pretty mellow and my wife is very easy on the throttle.

The car is broken in (9993 miles). My next change will hopefully be Redline 5w-20 syn.

 

Although I cannot state for certain myself, I was told by two separate Acura service techs that the main (and perhaps only) reason for switching from 5W-30 to 5W-20 from the 01s to the 02s was for gas mileage. Lighter oil = less friction on engine parts = less resistance = better MPG. Apparently this is why either oil can safely be used. The engine parts don't care, but your wallet will.