Same thing happen to mine rusty_wingnut not with the oil but just changing the air filter. Removed the old one which was black. At the time I was getting 38 ~ 42mpg once the new one was in couldn't get pass 36mpg but slowly it came back.

I've not touched the air filter, but Fuel Doctor did remove the pipework from the airbox, he couldn't get the clips back on so I told him to forget it and once I was towed home I put on some nice jubilee clips.

I wonder if one hasn't seated properly or if there is a small leak on this pipe causing my economy to fall off?