[C3H8]

most car company's say NOT to add oil unless it is BELOW the add line, just as Frank Lee said.

The reason for this, however, is not always clear. i know from personal expierence that most automobile oil pans can phgysically hold more oil then recomended without the oil level touching the crankshaft. Add to that, that when the motor is actually running, oil is being pumped through the engine and draining back, so the oil level in the pan will be less.
So why would they say not to add oil if not below the add line? It could be as frank lee said, wind age and the like. however being that the oil level in most engines is not even close to the crank, i would hazard a guess that they simply want to reduce the total amount of oil consumed. (if you add a quart a few days before getting an oil change, for example, the added quart is probably wasted). I think they say this so they can claim to the epa their cars use less oil. I don't think it has any part in fuel economy. All automobile engines use positive displacement oil pumps. As long as the pick up tube is submerged in oil, and the crank isn't whipping it up from grossly over filling, then you are good to go. It is only on splash lubricated engines, that oil level dramatically effects horsepower and fuel economy. for example, 5hp briggs and stratton racing go carts. People who run these things typically run them with as little as 16 to 12 ounces of oil. These engines get lubrication from a dipper on the bottom of the con rod which flings oil everywhere. This is called splash lube.

keep in mind this is only my guess. i always keep my oil topped up. if i am going on highway trip i will even overfill by a quart or so.