Also, the metals shed, because of the very narrow range sampled via spectrography, are limited in value. Sure, if you see a huge uptick it may warrant further investigation, but different formulas will have different "signatures" and certain base oil and additive chemistries may chelate material, which shows up as "wear metals" in a UOA, even though no actual increase in physical wear took place. In fact physical wear could be lower. Esters are a prime example of something that will increase metals in a UOA, but not wear.
Also, if we go to the extremes:
An oil that's capable of holding more in suspension than another may show higher levels than the one that can't. Varnish, sludge, these products are also full of whatever else was in the oil and an oil with a weak-sauce additive package that is saturated to the point of laying down deposits may show "better" than one that is more robust. This can also flesh out with a change in lubricants to one that has more aggressive chemistry and has better solubility as well as a more robust detergent/dispersant package when used in an engine that already has deposits from previous OCI's with a lubricant or lubricants that were not capable of those tasks. As those deposits are dissolved, this may artificially inflate the figures seen via a UOA.
A UOA can tell you if you have sufficient TBN remaining, but it doesn't tell you if you've reached the holding capacity of the additive package, and Blackstone doesn't provide oxidation, so there's also a lot of flying blind that can happen, which can be dangerous if the engine in question is particularly hard on oil, like the Honda example that @The Critic
shared in another thread.
First, let's limit our conversation to healthy engines. Equipment that is badly ailing and/or abused with no regular maintenance will not really help this discussion; I exclude those from my comments below.
Metals shed in a UOA are a representation, just as any sample is. If the sample is well taken, the representation is valid. I think those of us who undertand spectral analysis do agree that any particle above 5um isn't likely to be seen, and those that are are somewhat on a sliding scale because smaller particles vaporize with greater consistency than do larger ones. However, that "sample" is still valid. There is inference that can be taken from the wear metals; low values indicate a lower overall total rate, and vice-versa. Furher, as has been discussed elsewhere on this site many times, there are filtration studies that show excellent correlation between particulate loading and wear metals. Low wear metals in a UOA are echo'd in PC tests, meaning that (though the PC cannot see content), there's likely a lower percentage of large metals also in the sample. This is proven beyond any reasonable argument. It would be unrealistic to claim that a low wear metal count in a UOA is somehow hiding massive particles beyond the 5um limit in a normal, healthy engine. That's does not correlate in any manner with the PC studies regarding filtration efficiency. I will admit there certainly are particles of metals above the 5um limit, but that does not mean they are numerous or an indication of pending doom. We take samples of anything in the expectation that the sample gives us a relative view of the overall population. UOAs cannot see the entire metal count in a sump, but they can imply a relative rate of high or low metals overall. UOAs see content, whereas PCs see quantity; they can compliment each other and have shown good correlation between desirable and undesirable conditions of lubes realitive to the equipment they are in. It's true to say UOAs are "limited" in what they can see, but we have the ability to take samples and make an informed inference of the overall wear condition. But I disagree with you when you state they are "limited in value".
I agree completely with you in regard to the topic of esters and chelation. That's been seen quite easily in the GM 6.6L Dmax engines. They shed Cu like a dog sheds a fur in spring, when you introduce an ester-laden lube; that's because the oil cooler is a copper-plate design. However, you didn't also mention that the reaction of the metal to the ester will actualy subside over time as the metal normalizes to that chemistry. That normalization often takes many thousand of miles; often more than one or two OCIs. Which brings me to another point ... changing lube brands every OCI only compounds the issue. When we see folks trying Amsoil, then RP, then RL, and then continue down the oil-de-jour path every OCI, the engine metals never really get a chance to acclimate to the lube before the next is introduced.
But ... I would argue this ... how's that the fault of the UOA? How is that the fault of spectral analysis????
What that shows is a lack of understanding on the part of Joe Average regarding how his choices and actions can skew a test result. If you don't understand the benefits and limitations of a tool you are using, you're likley to misuse the tool, or misinterpret the results the tool provides. That is NOT a indictment of the tool; it's a clear display of ignorance upon the user. A UOA is only as good as the person using it, and frankly we have a lot of amateur hacks on this site grossly misutilizing and misinterpreting UOAs. But that's not the fault of spectral analysis now, is it?
To continue, I understand your comment regarding holding capacity of a lube, but that somewhat looks at the end of an equation and ignores the process unfolding. Just as engines shed metals at some rate, they also experience contamination at some rate. Sludge and silica don't just show up en-mass on day one; they accumlate over time. And the host oil starts out "fresh" with it's given load of anti-oxidants, anti-agglomerates, detergents, etc. Essentially, any oil starts out with 100% of it's ability, and then degrades. Yet the sump contamination starts out (near) zero, and escalates. So as long as the capacity of any one oil to deal with contamiation isn't overwhelmed, it's not really a a big deal to worry about how it fares relative to another lube. Let me show this as analogy; I think it helps to take things into another topic as an example.
- say you have two elderly adults living together in a small home. They produce dirty dishes, dirty laundry, and dirty floors at a rate which is easily attenuated by a maid coming in once a week to clean. The maid provides a service at a rate (once a week) that is sufficient for the job.
- now imagine if four grandkids come over for a week during summer; the rate of dirty dishes, laundry and floors becoming contaminated quicker will rise for sure. It may not be sufficient to have the maid come only once a week.
You have one of two choices, you can either increase the number of maids who come once a week, or you can have the maid come twice a week. Either of these would suffice for the higher "dirt rate". But OTOH, those higher cleaning rates are not necessary for the "normal" lifestyle of the adults living alone.
My point? If the rate of sump contamination is sufficiently controlled by lube X, it's not really necessary to use lube Y which may have "more" additives. If your OCI is a fixed value, and lube X has plenty of cleaning ability, then using lube Y (which has more potential) won't make a tangible difference in cleanliness; essentially that potential goes wasted. I'll make up values from thin air just to illustrate the example:
* if your engine experiences 10g of contamination every 10k miles, the rate of contamination is 1g/1k miles
* if the lube you chose has a cleaning agent capable of hold 15g of dirt, and you only run a 7k mile OCI, then you've got 8g of capacity that goes unused
* if you choose a different lube that has even more holding capacity (say the ability to hold 20g of dirt), and still only run a 7k mile OCI, then the unused capacity is now 13g.
Congratulations! If you spend more money for the "better" cleaning oil, but didn't use that capacity, you just wasted money.
Now, if you decided to lengthen the OCI out to 17k miles, the resulting 17g of dirt would overwhelm oil X, and therefor oil Y would be a good decision.
My point is that the topic of holding capacity of a lube is no different than that of an oil filter. If your product has sufficient capacity for the job, adding more capacity doesn't help; it simply goes wasted. You have to understand that "capacity" of any product has to be viewed in terms of the "rate" of exposure, relative to the total capacity of the product.
So while I do agree that some oils will hold contamination in greater quantity, that ONLY comes into play if the duration of the exposure exceeds the capacity of any one product and favors another.
Finally, it's true that BS does not directly measure oxidation, soot, etc. But what they provide is an "insolubles" count. That count is a visual reference of the relative opacity of the diluted sample. When we see values of .1 or .2 in a BS report, that indicates that the total insolubes (soot, sludge, oxidation) are very low; versus if the reading is .5 or .6 which would indicate a heavily soiled sample. I do agree that other means of measuring soot and oxidation individually are probably more accurate, but the insoluble count BS provides does have merit; it's not useless. Just like all the other topics of spectral analysis, the "insolubles" have to be understood in context.