Add a pint of Mobil1 fully synthetic and see if the warning goes away. If it doesn't, add another. A little too much oil isn't going to do any harm and it's far less harmful than too little oil.
From experience with the wife's 350 the difference between the full 8.0 liters (8.7 quarts) and the warning coming up is about a quart.
When it's time for an oil change (every 10,000 miles), just go to your local oil change place and have them change the oil and filter together, using Mobil1 fully synthetic engine oil. If you like you can order kosher MB oil filters (part no. 0001802609) and take them along. Whatever you do, make sure they replace all the O-rings that come with the filter. Otherwise you'll get a leak round the top of the filter housing.
0W30, 0W40,5W30 or 5W40 are all fine for typical southern US temperatures, but if in doubt ask your local MB stealership what they recommend. I use Mobil1 "High Mileage" 5W30 in all three of our Mercs. The "high Mileage" bit might be snake oil, but all our Mercs are high mileage, so I use it.