My 5 cylinder 300SD went from 24.95 mpg to 29 when I replaced the
broken odometer gear (the 12 tooth gear).

It had 10 teeth.
Funny, I was getting about 400 per tank, cruising speeds seemed okay.
I thought I was driving 66 miles to work each day.....

The 300D gets 25mpg consistently, odometer has no broken gears.
But that engine has no turbo.

Not having a 6 cyl engine like your SDL don't know what to expect.
But that timing of the injection pump is critical. Although, I have never
adjusted mine.

I too get baffled by some of the mileage claims. I used to be told that the
300SD would get 33mpg. Never seen that.

__________________
80 300D 340K Owned 30 yrs
83 300SD 440K Owned 9 yrs - Daily Driver 150mi/day
02 Z71 Suburban 117,000
15 Toyota Prius 2600 miles
00 Harley Sportster 24k
09 Yamaha R6
03 Ninja 250