Here is my opinion. I know someone who bought a '79 300SD new. They always used Castrol and still have the car. I use Castrol, although I was tempted to try the diesel oil at my local auto parts store. In winter below freezing, I use 10W40.
Diesels smoke when they are cold, and it is perfectly natural. What isn't natural is when people back up when they are behind you at a drive through restaurant. That happened to me. I had two problems:
A. My turbocharger seal was worn, and since they are oil cooled, a bad seal will cause the turbo to burn that leaky oil. I replaced my turbo and it made a big difference.
B. I had worn valve stem seals, and $10.00 and an afternoon sitting on the radiator was all it took to solve that problem of smoking at idle. Changing those seals is remarkably simple.
smoke would pour out of the tailpipe when I accelerated hard, and I was no longer able to drive anywhere without complete guilt and humiliation. If it gets that bad I would worry about. Until then, I think your oil usage sounds very normal. You found a nice 300SD I'd say.