5w50 is fine, but 0w40 or 5w40 is better. Shahnawaz pours 5w50 into almost all C180/C200 cars because they believe it will prevent leaks and prolong the life of the timing chain, but its not the case. Go to the Mobil 1 website and enter your car's details and you can see what they recommend. Use the UK website since the international/US website doesn't display the normal C Class line-up. I have been using 0w40 since the past 4 years in my C180K and it's been performing very well. Check if the oil filter was changed along with the oil. The oil filter requires changing during every second oil change, but most people do it every oil change. I've been doing it with every single oil change, but I've seen people at Shahnawaz do it at every other.
Check the spark plugs too, but you'll need to be able to pry the plastic cover open and then remove them.
68k miles is fairly low (110,000 kms), so there isn't much to worry about. The car has an adaptive TCU and when it's adapted to your driving style, it will automatically achieve good fuel economy. Driving in comfort mode helps too.
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