Engine swaps: turbo and later diesels for highway usability

Swapping in a turbocharged OM617A, OM603, or OM606 (usually paired with a 5-speed) is described as the common cure for the highway-speed problem, roughly doubling highway comfort and fuel economy. Commenters caution that bolting a turbo onto a stock NA 617 is not a simple add-on, and that a full engine-and-trans swap from a donor is more reliable.