First, YES, I've read all the different oil threads (and am beginning to be concerned that I seem to enjoy reading oil threads! )

So, I've read, ad infinitum, that, while certain oils (e.g. Delvac 1) can suspend 4% soot, MB cautions against exceeding 2% soot. So a lot of folks reason that we should abide by what MB says, regardless of what a given oil can handle.

My question: what's the logic of the MB recommendation? I mean, if the soot's in suspension, what could possibly be the problem?

I'm fine with going with what MB says....I'd just like to understand the rationale.

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14 E250 Bluetec 4Matic "Sinclair", Palladium Silver on Black, 159k miles
06 E320 CDI "Rutherford", Black on Tan, 178k mi, Stage 1 tune, tuned TCU
91 300D "Otis", Smoke Silver on Tan, 145k mi, wastegate conversion, ALDA delete

19 Honda CR-V EX 75k mi
Fourteen other MB's owned and sold
1961 Very Tolerant Wife

I don't know what MB's logic was, but my concern is that I want to minimize the amount of abrasive particles in my oil. Running clean oil has to be better than running oil with 4% soot in suspension. I cannot prove that higher than recommended concentrations of soot will cause damage, but I would prefer not to find out.

I think that's part of my question: if the soot is truly IN SUSPENSION, are there, can there be, any ill effects?

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14 E250 Bluetec 4Matic "Sinclair", Palladium Silver on Black, 159k miles
06 E320 CDI "Rutherford", Black on Tan, 178k mi, Stage 1 tune, tuned TCU
91 300D "Otis", Smoke Silver on Tan, 145k mi, wastegate conversion, ALDA delete

19 Honda CR-V EX 75k mi
Fourteen other MB's owned and sold
1961 Very Tolerant Wife

It appears to me that soot "in suspension" will circulate through out the engine, including bearings, cylinder walls, etc. The "in suspension" soot is not "dissolved" within the oil, we are still talking about solid particles (although very small) in the oil. IMHO, minimizing the amount of "grit" in the oil has to be a good thing.

yeah, but in an engine that has over 100,000k on it, the oil is probably over 4% concentrated after the first day or two of driving anyway so I think the 2% limit might only apply to engines were new when that was published..

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RIP: 80 300SD
RIP: 79 450SEL
2002 E430 4matic (212,000km)
2002 ML500 'sport'

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What makes you think that?

I respectivly disagree. Oil tests in the majority show soot or insolubles reaching the two percent level at about 3 k miles usually. Not always mind you.
So you cannot in general have 4 percent soot a day or two after a change. I also suspect soot levels rise quicker in cold weather as the engines have some difficulty getting to their maximum burn efficency. In my opinion they would be sooting at a much higher rate in the unefficient mode. To be fair it is also mentioned that the real hazzard occurs when the soot particles clump up.
Except for the exreme cost of synthetics compared to good dino oil brought in bulk this would probably be a dead issue. I also agree with guys like Larry. Change it hot and change it often. The other component is the cost of a rebuild on these engines. They are not chev 350s. For the cost of dino oil I would do the recommended changes in the spring and summer with it. If in a cold climate I also understand a comprimise with synthetic is going to be required. That unfortunatly is a no brainer.
The bottom line is less upside to putting an expensive oil in if you let it become abrasive while in there. Or let it drop it's viscosity rating. Both will shorten an engines life to some extent. Truly false econonmy in my opinion. Also a lot of you do not remember when oil was changed every 1000 miles like I do.
Certainly oils have been improved massivly with additive packages. Contaminates still accumulate in the oils with milage and the aditive packages wear out or are consumed. The one that really concerns me is the links that control the viscosity. A diesel or at least an older one does require the viscosity far more than a gas engine. The loading of the oils is serious business in a diesel.
I wonder if anyone has done the old fashion blotter test. You simply put a drop of base oil on a blotter. Every thousand miles drop another drop on the blotter. Compare one to the other. It used to show changes. Not particularily valid for what we want to know but it also does no harm as well. You might even be suprised how the oil responds to the blotter test.

When the blotter test starts to reveal a massive change in the oil it is time for it to go. The spread rate and extent of the blotter absorption is the best indicator that the base oil has changed. Particularily in the thinning and accumulated moisture and fuel contamination areas. Also you get almost color rings as the oil spreads in the blotter material. The above are just some opinions of mine right or wrong. I certainly am not interested in fighting over them.
Also I really try to keep an open mind. Since I guess I am a dino freak it did aid my cause about wear components in the oil. It was reported that there are no wear advantages between either as the components showing up in engine anyalisis are about the same for synthetic or dino oil at the same milage. I guess it's about time to find a way to buy synthetic in bulk for the guys that want to remain there. Or at least for us canadians as it is hard to get much under 25.00 for I american gallon of synthetic and sometimes higher at present here. I can buy dino in bulk with fairly good diesel ratings and approved for most new diesel engines at 6-8 dollars per american gallon from Walmart or perhaps canadian tire in 20 litre pails. You fellows in the states can do better on that price I believe as well.
The other reason I posted this was it does seem we will always launch an oil thread from time to time. As one poster also stated on this thread he fears perhaps for his sanity as he is also starting to enjoy them. Now where is that flame suit of mine?

That is the fundamental question. If it remains in suspension and doesn't clump, can it cause any wear in the engine?

We all figure that soot is abrasive, but, there's no proof that the size of the particles can have an ill effect if they don't clump together.

One of these days I might run the Rotella syn to 8K or so and see how high the iron levels will rise. Is it proportional to mileage, or disproportional? That would be the only way to determine if additional soot levels cause increased wear.

That is the question, and I don't think any of us really know. Personally, I believe less soot is better than more soot, so about 5000 miles is my "comfort zone." IMHO, oil is cheap enough that I don't have to optimize the process.

The SD is using about 2.5 quarts in 5K miles now, so, I'm tempted to push the change a bit. I once ran it up to 8K. The analysis didn't look so good in terms of soot and wear metals.

Might go to 6K and get another analysis. It's having silicon issues and I can't immediately determine the cause.

Don't get me wrong Brian, I admire your analytical and scientific nature about this, but if it were me, I would not use my own vehicle(s) as Guinea Pigs. There must be a better way to answer the question.

Actually, I consider 2% to be a high soot level. In the course of a reasonable OCI, only an engine in less than good condition would exceed this level.

My $0.02,

Larry, I agree that 2% is definitely the upper limit. But, the SD has been running less than 1% on all of it's 5K changes. The makeup oil surely has an effect on this. We're quite conservative on the 5K changes with the synthetic oil, especially when makeup oil is necessary.

All else staying the same, as soot/solids go up, so does the viscosity of the oil.

Given that the oil is already too thick for optimal folow/lubrication at any temperature below 180F, I would consider further thickening a negative.


Tim

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2009 VW TDI Jetta Sportwagon 172k miles (rear-ended harder than Elton John on 8/4/13. Total loss)

1991 Volvo 240 142k miles (T-boned by a stop sign runner. Total loss)

Part of me wonders...and this is sheer speculation...if, when MB originally made that 2% recommedation, the limit of a decent oil was around 2%. No point specifying a limit that no oil could handle, right?

__________________
14 E250 Bluetec 4Matic "Sinclair", Palladium Silver on Black, 159k miles
06 E320 CDI "Rutherford", Black on Tan, 178k mi, Stage 1 tune, tuned TCU
91 300D "Otis", Smoke Silver on Tan, 145k mi, wastegate conversion, ALDA delete

19 Honda CR-V EX 75k mi
Fourteen other MB's owned and sold
1961 Very Tolerant Wife

It appears to me that the soot limit is independent of the oil quality. When I had M1 analyzed after 10K miles, the TBN was still about 6, but the soot was getting high. I do not believe that oil brakdown is the limiting factor in these engines.