Impact Of High Oil Level
 

After my last oil change, I noticed that the oil was maybe a cm (3/8") above the full line. How much of an impact do you think that would make on most vehicles?

Depends on design... If your crank ever goes below the oil line (or is immersed) - you'll have more internal losses as now you have to push through a much more viscous substance (as compared to air).

I had pepboys change my oil while it was in for inspection last year. They did the same thing, I checked to make sure there was oil in it but I didn't check the oil level until a week/tank later. That tank I lost 3.5mpg on a 97 civic that was getting 38.5-39mpg.
I checked the level, drained out over a half quart of oil.
2 weeks ago I went to the Honda dealership and got my free tuneup from the epa settlement for 97 honda civics.(it extends emmisions warrenty to 12 years or 150,000 miles and they threw in a tuneup also. I had a free o2 sensor installed.) They did the same thing, I guess they don't get all the oil out, but give it a full fill wothout checking the level. I got a half a quart out again but this time I checked the level when I got home instead of a week later.

sipnciv, they actually filled it perfectly. I very stupidly underestimated the slope of my driveway. With the dipstick so close to the rad end, it really gets affected by the tilt more than any other vehicles we've had. I checked it at a level parking lot and it was right on the F line which is where most people would expect it after an oil change. Next time I'll ask for it midway between the marks.

Thanks...

Read your manual. On alot of cars, they can be full and nowhere near the full mark. It depends on the model.

Here's a general way to tell how overfull it is. In most cases, from the low mark to the high mark is 1 qt. So, you can judge what a 1/4 or 1/2 is. So without seeing it, your probably 1/4-1/2 qt. over..