Also, the consumer's reports. What was the average testing conditions performed? A diesel gets similar highway mileage to city driving.

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1987 300SDL (324000)

1986 Porsche 951 (944 Turbo) (166000)

1978 Porsche 924 (99000)

1996 Nissan Pathfinder R50 (201000)

A W210 E320 should get close to 30mpg on the highway.
A W210 E300D should get low 30's on the highway.

Lets say 28 gas vs 34 diesel, so 6mpg difference. The M116 V6 is a dirt cheap engine to run unlike the 606 that can break off glow plugs in the head.
The M116 only requires spark plug changes every 100k miles or 4 years, their are 12 of them.

On the resale end of the market 6 years down the road you can get a much nicer E320 with half the mileage for the same E300D.

Their both good engines take your pick. The new CDI is a very quite car you have to listen to it to be able to tell it is diesel. The New 3.5 liter V6 in the W211 is a far better engine then the M112. It now has 4 valves per cylinder and 4 overhead cams. (like the old M119 ) I have never driven one but supposidly it is a sweet engine. Take your pick the choice is between best and best.

I would kill for an S400CDI though. I was watching a video of one on the autobahn, the speedo was way over to the right. Doing about 155 converted.

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1999 SL500
1969 280SE
2023 Ram 1500
2007 Tiara 3200

For the long haul, this new filter seems ominously expensive. Ceramic anything is expensive. If the design life of the cannister follows the rest of the current MB practice, it will have to be replaced at some point and most likely right after the warranty runs out. And if it interfaces with the electronics with all that self cleaning and monitoring BS, then there is no bypass opportunity. My 210 looks better all the time.

The high pressure fuel injection concerns me regards to leaking tubing. The 210 is marginal but manageble.

First of all, you can't know everything for yourself. You have to believe what you read at some point. In fact, at one point or the other, you have to completely entrust yourself in the hands of what you read. All you can do is try to read from the sources that seem as credible and unbiased as possible. In this case, I gave you both EPA mileage figures (which are rubbish IMHO) and Consumer Reports figures which are real-world driving figures. Consumer Reports has nothing to gain in the entire process. By that I mean that making the diesel get better mileage in their tests will gain them nothing. Given that, they are the most neutral source to me.

So you actually get to hear a new version of every car you buy? I don't believe that. But if so then good for you. I don't wish to go to such lengths. I have seen the results of Consumer Reports' tests excellently match my own results from many cars I've owned over the years: Toyota Avalon, BMW X5, Mazda 929, etc. So I trust them.

When you say a diesel gets similar highway mileage to city mileage, what diesel are you referring to? Firstly, my diesel definitely doesn't fit that statement. Secondly, these CDI engines are totally different breed of engines. Many old principles of the IDI diesel engines don't apply to them in any way. Check the UK CDI Boards? Have you ever been to Britain or know anything about Britain? Well, they have way more traffic that we have in the USA (on average; cities like LA obviously are traffic monsters). Their driving habbits are completely different. And also, as you read fuel mileage figures off those sites, remember that their gallons are different from US gallons.

I'm not going to say you're wrong when you said you've heard a brand new MB IDI diesel engine. I've never heard one so I can't say - however, I remember reading a comparison of sound levels for different luxury cars in a 1986 or 1988 issue of Car and Driver that I have. It will be huge task to look for it and as a result I won't bother, but if I ever come across it, maybe it will explain things better.

Finally, all those old diesel engines may get all that excellent mileage because they weren't subjected to today's strict emissions requirements. I wonder what kind of mediocre mileage those engines would get if they were to be made to meet those emissions requirements. Hmm..

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1999 Mercedes-Benz S600, 103K miles - garage queen
1988 Mercedes-Benz 560SEL, 89K miles - daily driver
2007 Hyundai Sonata Limited, 31K - daily driver