(Reportedly) from a Redline Chemist waaaaaay back in 2009:
"Unfortunately, oil analysis is not very good at distinguishing wear between different formulations. Emission spectroscopy has a particle size limit of 3 to 5 microns, which means that particles larger will not be detected. Unfortunately, most serious wear issues generate wear particles in the range of 5 - 15 microns. Oil analysis only measures about 15-20% of the particles in the oil, and changing form one formulation to another is likely to change the particle size profile. Usually formulations with more antiwear additive will more aggressively react with the metal surface and when rubbing occurs will produce smaller particles. Generally, more antiwear additives will give greater iron spectrochemical numbers, even though the total iron can be lower. There are other techniques such as ferrography, which looks at the wear particles under a microscope, but now we are talking about analysis many times more expensive than spectrochemical analysis. The oils with the better spectrochemical numbers will be much less chemically active on the metal surface, so they will be less able to handle more severe loads. There is always a trade-off between chemical wear and adhesive wear. Chemical wear is the very small particles and soluble metals which is identified in the spectrochemical analysis, while adhesive wear is many orders of magnitude greater than the chemical wear, but much is not identified in spectrochemical analysis. But if you were using spectrochemical analysis as a maintenance tool and started seeing a deviation over the baseline, then you would know something was wrong.
It is very difficult for an individual to be able to look at numbers which will conclusively determine the best formulation, you simply have to rely on the reputation of the marketer and whether you trust the marketer's technical expertise. With most of our formulations, we rely on major additive manufacturers to do the basic API sequence testing to determine criteria such as antiwear, dispersancy, cleanliness, etc. All the oil companies rely on the additive manufacturers to do the engine test work. We will take their basic package and add additional antiwear, friction modifiers, oxidation inhibitors or whatever can be safely modified to provide superior performance. Some of the bench tests such as 4-Ball can be useful, but a blind adherance to optimize with one single test will result a less-than-optimum performing lubricant. There are always trade-offs in engine oils, and we try to enhance antiwear and friction reduction at higher temperatures and loads, while trying to maintain performance at lower and normal loads and temperatures."
