I have an old 78 300D with 352,000 miles. It has had noticable blowby for the last 100,000k miles. So long as I keep it under 60 mph it does burn that much oil. I did find that using the proper diesel rated oils slows the oil consumption. (DELO) .
Doesn't seem to get any worse. I have read here that the pistons on these old diesels are likely to wear the cylinders oval.
Have a much lower mileage 300D engine and trans sitting in my garage, hope to install them later this year. But until then I just check the oil weekly and add a little as needed.