You didn't say how many miles / kms on this engine. Engines use lube oil
for three reasons: leaks, wear and lack of maintenance.
Leaks are just that, look for clean oil stains on the engine and
underneath - by clean I mean more oil than dirt indicating a large
enough leak to carry away the dirt.
Wear of the piston rings is inevitable in any engine, good maintenance
simply allows the rings to last longer. A compression test will tell if
the rings are worn, if so, an engine overhaul is needed - such an engine
probably has done 250K miles / 400K kms.
An uncontrollable oil user is worn valve stem seals; these plastic seals
wear out and allow lube oil to be sucked into the intake manifold. Some
engines have this problem much sooner than others - it's quite variable.
An experienced M-B mechanic can replace the valve stem seals without
removing the cylinder head, a job that costs about US$500 in the US.
Some engines suffer from lack of maintenance - specifically, oil
changes. The inside of the engine gets dirty and the piston rings no
longer seal as well as they should. The cure is a few frequent oil and
filter changes, say every 5K km and some long trip driving so the engine
really runs at a good speed for some hours.
Most auto manufacturers will tell you that 1 ltr of oil consumption per
1K km is "normal." I don't but I expect 1 l. per 8K to 10K kms from a
relatively new car. The harder a car is driven the more oil it will use.
If your car has done over 150K miles / 250K kms some oil usage is to be
Oil you buy has specifications. The low SAE numbers, 0, 5 & 10, indicate
very thin oil and the higher numbers, SAE 40 & 50, thicker oil. You
should use SAE 10 - 30 in the winter and SAE 20 - 40 in the summer.
Don't use very thin oil in this old engine.
Finally, don't confuse water vapor in the exhaust during cold weather
with oil burning. The smoke you see upon hard acceleration is mostly
soot being blown out of the engine and exhaust pipes.
I hope this helps you to understand your engine's needs, not only this
one but those that you'll buy in the future.