I had a '79 280E for four years and it always used oil in the range that yours does. The word that I got from several respected sources was "it'll go for a long time like that" and unless the performance was affected to just add the oil. I bought mine with 97K and drove it another 35K without oil consumption increasing.
I used Castrol 10-40 in the summer and 10-50 in the winter.
My advice is to save the money, add the oil, and enjoy the car.
Falls Church VA
'86 300E 5 speed
'95 C220 (wife's car)
'98 Porsche Boxster
Past: '79 280E, '82 300D (18 yrs), '77 240D,