Acceptably Safe Soot Levels: Can Anyone Speak with Authority?
 

First off, don't be put off by the title....I don't mean to exclude those who CAN'T speak with authority (for then I would have to exclude myself).

Everyone agrees that AGGLOMERATED (BTW, a great vocabulary word to toss out in casual conversation... "You kids need to stay together and agglomerate so I can keep an eye on you.") soot is bad/evil and must be avoided. This, as far as I can tell, is a function of the quality of the oil. Delvac 1, for example, can handle at least 4% before agglomeration (clumping) occurs.

However, apparently even NON-AGGLOMERATED soot can cause damage (this on the authority of one of the guys in the lab that did my oil analysis).

The question is: what level, for all practical purposes, is perfectly safe?

At least Marshall Booth (well nigh the patron saint of all things Mercedes diesel) used the figure of 2%....and I think Mercedes published that figure at one point, but now I'm not sure. So my operating assumption has been that, as long as I'm well under 2%, I am, oil wise, doing what I can to maximize my engine's longevity.

But, there appears to be a school of thought that ANY amount of soot is harmful and that, therefore, it should not be allowed to rise above x%. I've seen 1% as the safe threshold. But where does that come from? And how is it not arbitrary? Why not 0.5% or 1.2%? If ANY amount is harmful, then why shouldn't we change our oil every 1500 miles or whatever? I like to save money as much as the next guy, but I'm really willing to spend whatever I have to on oil and filters to ensure extremely long engine life.

Honestly: If thought that changing my oil every 1,000 miles would promote engine longevity, I would do it. But I have no reason to believe that.

Currently, my plan has been 10,000 mile drain intervals, which would equate to about 1.4% soot. If that sounds like a risky plan, could someone please explain to me why it is risky?

Thanks in advance for contributing to the discussion and helping me to grow in my understanding of these things.

I think people who care go with the Mercedes Benz recommended change intervals for their severity of driving, change the filter every time, and use a good brand diesel rated oil. And leave it at that.

Maybe try your 10000 plan and have your soot analyzed for the first few. If low, continue.

That is unquestionably the safest approach. But, I'm assuming that those who are intrigued by the idea of extended drain intervals would not want to do so at the expense of their engine.

On my 92 with 225,000 miles, my oil analysis came back with just over 1.1% soot after 7750 miles. Some would gladly drive another 3000 miles; others would be nervous by that level of soot in their oil.

Shertex, you don't say what brand, etc., of oil you are using. Clearly, that matters, whether you use a synthetic or non-synthetic. Diesel-rated oils are capable of absorbing soot, the amount varying between brands and types. In addition to the miles on the oil, you need to consider the length of time the oil is in the engine and the amount and type of use that the car gets. Anyone attempting to push oil beyond the factory recommendations might be wise to have the oil analyzed at the mid-point.

Jeremy

I use Mobil 1 Turbo Diesel Truck 5w40.

Something else that I'm thinking about is that, at least on a 124 and 126, you still have lots of oil in the oil cooler that you cannot get out unless you disconnect the lines, so the oil will never be really clean. In my case, I just changed my oil last Saturday and today is black as ink. I don't know the percentages though. When i get a garage to work in I would probably consider disconnecting those lines to get out all the dirty oil...

As Marshall Booth used to say, the blackness is a feature, not a flaw....just means your oil is doing its job. He also made the point the human eye has great difficulty discerning different degrees of blackness. The fact that the oil "looks" jet black almost immediately is no cause for concern.

Agreed. The risk of damaging the oil cooler and/or its lines does not justify draining that last pint of dirty oil and even if you did, the new oil will turn black almost immediately.

Mobil-1 TDT 5W-40 should be good for 10,000 miles, especially if you do an analysis occasionally. Our '96 E300D (W210, OM606NA) has Mobil-1 TDT 5W-40 in it and is driven about 8,000 miles annually. I change oil and filter once a year. The oil analysis typically comes back with "low" soot (Associated Laboratories doesn't provide a % number) and otherwise looks pretty much like new oil. I just changed the oil and have a sample on my desk waiting to go off for analysis.

I don't see any reason to limit yourself to less than 2% if MB recommends a 2% max., M-B is usually conservative and when the 603 was designed, most specifications were straight from the Engineering department and well within limits.

Sure, less is better. The reason that some of us run high-grade synthetic oils is because we want better than the minimum standard, seems to be a contridiction to run it to the max. allowable soot loading.

If you want to keep your soot loading to a minimum, you'll be driving with a barrel of oil in the trunk and only lubricating with virgin oil. Your engine will without a doubt last longer. Since I've torn down a couple of 250,000mile 603s with normal maintenance and witnessed crosshatching on the cylinder walls, un-scuffed skirts, zero turnaround ridge, and excellent journals I have a hard time believing that running with good quality oils and normal soot loading is a problem.

I also am interested in what the maximum recommended soot loading is and how it was determined.

So what sorts of soot loads have others who have done oil analysis seen? On one 602, I saw 0.8% at 189,000 miles after 7500 miles; then 1.1% at 225,000 miles after 7750 miles.

Within the next few weeks, I'll analyze the oil on my other 602 at 94,000 miles after 9500 miles....that should be interesting.

I don't know the basis of the 1% or 2% values either. I do know that I'm not going to exceed the manufacturer's recommended interval for oil changes. It is interesting to have the occasional oil analysis done to see if anything nasty is happening, but I wouldn't use it to justify extending the interval.

Of course, the recommended interval must be based on certain assumptions about the likely characteristics of the oil at that mileage (in my case, 5000 miles). It's hard to imagine that a top-of-the line oil in 2010 won't significantly outperform what was available in, say, 1992. That would seem, at least potentially, to warrant longer drain intervals....

UNLESS the soot level, irrespective of the kind of oil that's used, typically hits a certain limit at 5000 miles. I think that's part of the logic of those who are uncomfortable with longer drain intervals: soot, even in suspension, is bad for the engine, thus we should always stick with the original recommended interval.

FWIW, here's a note from someone who works at the lab that did my oil analysis, explaning why he would stick to a 1% limit if he were in my shoes:

I would shoot for less than 1% on an automotive engine as the sump capacity is smaller and the potential damage with high soot. If you look at the sample we have been discussing you can notice a common pattern. Your previous sample had a low value of soot and the lead and iron values were much less than this sample of lead and iron. Lead is always from the rod and main bearings and generally lead/tin/copper are the bearing layers. The iron can be several places but it typically will elevate with soot. You are being very proactive with your vehicle maintenance and I just try to help with any advice i can provide with a "fellow gearhead". I would keep with the Delvac 1 as this is my product of choice with all my vehicles, gas and diesel. Keep the soot to less than 1%, (which could be excess idling, lots of local driving, or intake restriction), keep up with Delvac 1 and continue sampling and I see no reason why you can't get 400k plus.

That is essentially my thoughts. In old diesels, the oil will have a large amount of soot long before it chemically breaks down. I don't know how much is "unacceptable" but I doubt anyone could develop a single value. If we accept that any amount of suspended soot will cause some increase in wear, any "acceptable" limit would have to be based on an acceptable rate of wear for critical parts. Without any better data, I'll just stick to the manufacturer's recomemdation of 5000 miles.

I recognize that having a short interval due to soot loading somewhat negates the advantage of using synthetic oil, but I still think it's worthwhile for cold starting.

Is he saying that he can detect an increased amount of bearing wear at higher soot levels?