You aren't burning the oil, it's leaking out under load. If 20W 50 causes oil spots, the ligher oil is running off the engine at speed, doesn't leak at idle, so the oil drips off before you stop. Higher viscosity oil is still dripping when you get in the garage. A drip every couple minutes is a quart in 1000 miles or so!
check in the air cleaner housing where the large blowby tube is -- is it dry?
How old is your oil filler cap? My TE was using far too much oil (a quart in 1000 miles or so) -- I've changed the valve cover gasket, was expecting to do the front cover, and discovered that the valve cover gasket was oily again. So was the area around the oil filler cap. Gasket on the cap was rock hard and premanently dented where it is supposed to seal, so on the highway, oil oozed out. Ran off the engine, left occasional drops on the drive. New oil filler cap, so far on oil useage.
Other things to check are for oil in the sound insulation tray underneath, and for a drop hanging at the bell housing (rear seal). I assume you don't have the front cover leak so very common on the M103 since you just had the head done. Make sure the PCV purge hose (the samll one between head and intake) isn't plugged.
Still, take a look at the plugs. Crusty residue on the center electrode at relatively low milage is and indication of burning oil, you will then have to find where it is getting into the engine.
Unless the valve guides have excessive clearance, it's not going down there. You also don't have the valve cover filling up with oil, either, it goes down the chain case.
I'd lood for a leak somewhere on the block that runs off into the sound insulation panel.
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!