I had to add 1 quart of oil to my 78 300D the other day. I changed the oil 1,100 miles ago.
This seems exessive to me. The car has a new top end done by the dealer here. (rebuit head, new cam, timing chain etc..) 144,000 mi on origional lower end.
It does seem to smoke a lot on first start up, yes it is blueish.
I average 22 mpg. I travel 20 mi a day total, 2 miles in stop and go traffic.
I have used fuel additives before. It seems to help smooth the engine out and I guess improve fuel economy a little.