The first thing I would recommend is to place the car on a disciplined, rigorous oil consumption check. Check it frequently, consistently, and record the oil level and mileage everytime it's checked. The difference between the full and add mark will be explained in the owner's manual as will how to check the oil. Read it!

Most manufacturers recommend checking the oil with the engine fully warmed up a few minutes after shutdown, and the car MUST be on a level surface. A good way to follow this advice is to check the oil at fuel stops after you have completed fueling. Most pump island pads are level. Prior to fueling I remove the dipstick, wipe it down, and lay it on the cowl. This allows the oil to drain out of the dipstick tube, so when I insert the dipstick, I get a clear reading.

Once you are confident that you have an accurate data set that allows you to compute actual oil consumption, begin looking at causes if it is excessive. In my experience, most drivers have only a vague idea of their oil and fuel consumption because they don't keep accurate records, don't check the oil using a consistent process (or check it on unlevel surfaces) and end up with little more than a vague guess, which is often far off the actual mark.

Also, shortly following an oil change, oil level should be checked to be sure it was filled to overcapacity. Many "quicky" oil change joints don't use the proper oil quantity and some have reported that even MB dealers have under or overfilled their cars.

Although a quart every 800 miles seems high for the mileage, this rate of oil consumption will likely not cause any problems other than the inconvenience of checking and adding oil. Cars with functioning catalytic converters usually don't smoke because the oil that is not burned during combustion is oxidized in the converter, and as long as the car passes any required emission tests it's easier to add oil as required rather than tearing into the engine to find the cause, but a fuctional check of the PCV system is a good idea because it is mostly external and any problem should be relatively easy/inexpensive to correct.

Replacing valve seals, which is a common cause of oil consumption, is often a fix, but unless/until the oil consumption gets down to 300 miles per quart or about one every tank of fuel and you want to keep the car, it's a lot cheaper to add oil as required. There will probably be no other adverse affects if the oil consumption is at the rate you report.

Duke