I see the same sort of thing. I believe that the oil tends to thin out after a while and is more easily burned or thrown out the vent. This may indicate that a 2,000 mile oil change would be worth considering.
I have an old engine in my car while I rebuild my good one. I'm not too concerned about the engine in it now. But, I am considering 2,000 mile oil changes when I get my good engine finished.
Hope this is of some interest to you,
'84 Euro 240D, 516K miles
'88 300E 5 Speed
'81 300D Daughter's Car
Over 800,000 miles in