My 2007 Honda Fit requires 5W-20 oil, but I recently changed the oil and put in 5W-30 by mistake, since that's what my other car takes, there was a special at the auto store, etc.
The engine seems to run fine, but I have noticed a slight decrease in mileage -- I usually get ~30 mpg (combined hwy/city), and in the one tank of gas I got since the oil change, I noticed it was about 25 mpg.
Should I drain the 5W-30 and replace it the correct weight? Am I just looking at reduced gas mileage, or could I be causing damage to the car (through oil deposits or whatever) because it's not engineered for the heavier weight? And what's with oil weights, anyway? I thought the lower number was the most important one, in terms of startup viscosity, etc.