I recently purchased a 1992 300D 2.5. I have been getting about 35mpg highway. Is this reasonable?

I drive 50-60K a year and want to make sure I am doing all I can to maximize my fuel economy. A few questions:

- Are these engines to have their valves adjusted?

- Are there performance chips for these diesels on the market that have shown increased efficiency?

- I have had a lot of luck with synthetic lubricants before. What are your experiences with a change from mineral to synthetic all around at 136K. My primary concern are leaks developing.

- What else can be done to increase fuel efficiency?

truth