Hatterasguy has a great point, I think MB got it right with oil consumption on the 603 engines
How often are you changing oil? And what brand of filter? (Oboy, I don't want to start another filter debacle, its just that the choice of filters is less with the 603. And since I change about 5K intervals a good filter is necessary.
I own two '87 300D turbos and both show around 265K miles. One has new rings which are just about broken in actually. Both have #14 heads (rebuilt) and amazingly one uses about a quart every 4K and the other (with new rings) a little less than that! Neither have any significant leaks, just a few drops off the IP shut-off lever shaft which I am too lazy to replace the O-rings on.
I believe the turbo bearings let some oil pass into the intake I think that's where most of it goes (my EGRs are in the test mode)
I use AMsoil 15W40 and change at 5K even though the last analysis showed I can go further. I also tend to check whenever I am looking for something to do, unlike the 617's which I religiously check every other fuel stop and wind up topping up inevitably.