I just bought a c240 for a bargain of $500. However, it was running pretty rough (rough idle, no misfires or smoke). So I took it to get an oil change as I suspected it hadn’t been changed in at least 13k miles per the carfax. The shop said that they wouldn’t do an oil change because the oil cap had a milky residue on it and to bring it to the dealer because it’s probably a blown head gasket. I did some looking around on here and found that it could either be a blown head gasket (which I hear rarely happens on a w112 motor), a broken o ring for some timing module, or just that it was in the cold and never went on a long trip. The guy I bought it from only went about 20-50 miles every couple days and I live in the North east where it regularly hits single digit temperatures in winter. I was thinking it was that, but the milky solution smells like a marker. Which I suspect to be coolant, not just water and oil. Last night, I put in three quarts of 0w-40 Mobil one oil and all of the warning lights except the check engine light went away. I noticed it’s running smoother, but still has a rough idle. I have an appointment with my local mb dealer to get it checked out Friday, but I was wondering if I could get some thoughts on here. There still aren’t any misfires, smoke or engine noises. There a ticking sound but it sounds like its in the cabin side of the firewall