carock,
The current technology can count particles, but most of the UOA report is generated by stripping the elements to plasma and looking at them directly by their signature spectra. As such, the smaller the particle, more likely it is to be fully measured. In addition, the larger particle are often (hopefully) captured by the filter to never be seen in a UOA.
My largest issue with UOA to asses an oil is that I really think that a UOA (in the absense of mechanical failure) is really measuring the efficiency of the airfilter/breather system. Within the normal range of elemental analysis, wear seems to more correlated with Si and/or engine type than it is related to oil...by far.
So, I think that if oils are little different, you will never know. If they are very different, then large data sets should show the differences.