I will be polite as I can. Dealerships don't care. Oil is oil to them. I bet you anything when they gave you free oil changes they didn't know they gave away 81/2 qts of oil rather than 5. How do you suppose they will recoup their loss? 5w-30 or 5w-40 if you can't find the 0w-30 in a synthetic is what is recommended along with fleece filters. If you don't use these then you are wearing out an engine before its time. Nothing is free. Pick up the oil and filter and take it to a local shop. Keep in mind most local shops don't do topsider oil changes. So they may have to learn as well. Don't be a cheap skate with the oil. That 100 bucks at an MB dealership is cheap insurance if you don't want to do it yourself or teach a local shop to do it.