I've got 250,000 miles on my '99 Ml-320 and it is getting towards using a quart of mineral oil (always Castrol 10-40) every 5,000 mile change (inc/ filter). Bit of smoke in the air after I fuss around launching the boat.
This thread has reassured me that it is unlikely that a sudden increase in oil usage is in my immediate future.
By the way, replacing the oil pressure test port (like $20) cured an oil drip problem many miles ago. Some may mistake this for usage.