Logic of MB 2% Soot Limit
 

First, YES, I've read all the different oil threads (and am beginning to be concerned that I seem to enjoy reading oil threads! :dizzy2: )

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.

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?

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..

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.

What makes you think that?

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.

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,

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.

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.

oil replacement
 

Brian, I would think with the 2.5 qts of oil used and replenished your additive package is kept at a suitable level. I personally have gone to 8k mi in my Powerstroke with it using one quart at about 3500mi and I change the filter at 4k and end up replacing 2 qts+ at that time. Blackstone says I could go to 10k if I wanted to. Having said that, I think the soot levels are a bit less in that engine. However, you might try that and see where it goes with the analysis. JAI(just an idea):D
Bud

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

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?

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.