Is it common for a decrease in MPG after switching to synthetic motor oil?

I've noticed that I've gone from about 26 MPG to ~21 MPG mixed city/highway driving AFTER I went from conventional to synthetic.

No other changes, no mods, same tires/same pressure, same driving style.

I had a bad cat that was replaced by the dealer prior to the change in oil but I don't know how that would effect my MPG. I also replaced the air filter with a brand new one (Honda, not aftermarket)..but this was after the change and after I noticed the decrease in MPG.

Car had ~32K on it when I switched to synthetic (Valvoline).

Any thoughts?

Thanks

 

Oils of the same viscosity share the same properties, synthetic or not. Further, almost all off the shelf synthetics, Valvoline included, have group III base stocks just like conventional. The switch to winter gasoline would have more of an effect on fuel economy.

Get your codes pulled free at Pep Boys and see if something is going on with your O2 sensor. Fuel economy will go down if your timing is being retarded.

 

The S isn't throwing any codes. A bad O2 sensor should throw a code, correct?

 

Right.

 

Switching to synthetic of the same viscosity should not adversly affect gas milage. Something else is happening.

 

if you have too much oil, your gas mileage will suffer

 

if you were to change up or down one full viscosity level the maximum difference in economy and power would be about 2%, so I think it is fair to say that something else is happening there. Where do you live ? , winter fuel blends would account for that difference if you live in an area that uses winter fuels.

 

I disagree. I think this is a bit dramatic for a simple change to winter blend gas. I would expect no more than a 15% change in MPG.

On that same note I can't say that I have a better explanation to offer.

 

you could try resetting your ecu?

 

How many tanks have you measured after the switch? I'm not entirely convinced that it's causal.