I am driving a ml320 with 112000 miles, and using no oil as well.

Using 15w40 grade (conventional) motor oil. < this is a
hvy duty truck oil spec SL/CI-4 >

I have an associate with a ml320 who used synthetic and had
higher consumption than with 15w40 motor oil.

The vehicle did consume some oil approx 1 x qt per 2000 miles
with 15w40, but way below that of synthetics.

Being in the petroleum business (a mfg of motor oils/lubricants),
i note synthetics do have a higher consumption than
conventional motor oils. ( this conclusion is from feed back
form hundreds of repair facilities of all types: auto, marine,
hvy duty trucks, and as well as hvy ocean going vessels. )

We ship tens of thousands of gallons per month of motor oils of all types, and i note cross country and local truckers DO NOT
use on a normal basis... these trucks are logging up to and some over 1 mill miles per engine. * and note most burning high to mid sulpher diesel fuels.

Yes, synthetics are great oils, but conventional oil can hold there own if the vehicle is maintained properly.