Hello all, I'm considering purchasing an '83-'87 300SD (I believe that is the correct lettering) large four door sedan.

I noticed around '85 was the last year for the ol' 5 cylinder diesel which seemed to be the fav motor but what about the later 6 cylinder motor? Not the 350 one but the 300 series.

Would there be an advantage to going with a 6 cylinder diesel motor for the 4 speed auto and perhaps slightly more HP/TQ stock?

Could an obvious MPG gain be had with a 4 speed auto and the 6 cylinder over the 3 speed auto and the 5 cylinder?

Opinions and info are most appreciated!

Lots to cover, no time. I suggest you use the search feature here and locate lots of discussions. The after-85 SD models are now called SDL models, L for Lange (long) wheelbase. The 300SDL is a 6 cylinder diesel, was sold in the NA market in '87 and is a very good running yet economical car for its size. Fuel economy for the 6 cylinder diesels is better than the 5 cyl engines of the 300SD because of improvements.
The '85 was the last year of the cast iron head engines, and some feel was the best most reliable diesel. I own 300D of both engines and feel good about both designs, but should make you aware that there problems with the 6 cyl engine that can be expensive (read about cracked head syndrone in OM6903 engine for example.) I have not exerienced any problems with mine.
There are vast differences between the cars besides the engines.
Finding either of these cars in good condition is the challenge these days, with so many high mileage cars on the market, the "right one" is getting hard to find.
Buying from a Shop forum member (see the Forun Jump and select Shop Swap, Cars) is a good way to avoid the Ebay syndrome (mileage roll backs, sellers who lie, deceive, etc. YMMV.)
Good Luck,
Dieseldiehard
1971 220 (gas) 4-spd manual 106441
1979 300TD w/ ’85 turbo engine 295530
1983 300D 243280
1985 300TD 217300
1987 300D 258230

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'95 E320 Wagon my favorite road car. '99 E300D wolf in sheeps body, '87 300D Sportline suspension, '79 300TD w/ 617.952 engine at 367,750 and counting!