What exactly separates the 'SD' from the other models? A buddy of mine has an 81 300D and we're trying to determine what exactly the differences are in terms of engine/drivetrain so that we can be better prepared for working on it. He's becoming pretty familiar with his car and we're not sure how much of that knowledge can be applied to mine. Does anyone have any insight?

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1993 Mercedes-Benz 300D