Do not use just any old oil. Make sure that you use a diesel oil that is designed to suspend soot, such as delvac, rotella or dello.

300,000 miles without service isn't impossible.

What? We are talking about the 617 here? I would say that 300,000 is ablsolutely to be expected. 5 to 600,000 is realistic. 900,000 to 1 million isn't impossible.

It seems to be typical of MB engines to require a valve job around 200,000 miles, at least on the gas engine. Diesels may or may not last longer, depending on mainetenance and service.

617's absolutley do last longer, and they are not typical MB engines. You can't compare the 617 with any of the gas engines or later diesel engines for that matter. It's an entirely different sweet beast.