My guess is that you are leaking it rather than burning it, unless it really burns clean!
Check the spark plugs for heavy ash deposits. If present, you are burning the oil.
Check the vent hose to the air cleaner, the large one on top, for oil. If present, you have a blowby problem, if not, it's either leaking it out or sucking it down the valve guides.
Vavle guide wear can get so bad the idle mixture goes out of control from air leaking down the guides, will drive you nuts, but this usually only happens at greater (150,000) milage.'
The oil really runs clean in this engine, so you may not notice leakage on a gravel drive, it won't be black at all.
1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!