Pete,
It all depends on what you want to find out. BuckHntr in his reply roughly described a simplified method of assessing the amount of wear, assuming that this is what you are trying to examine. However, there are numerous other goals that could be achieved and information collected, which would call for different extraction method(s). For example, one of many goals would be to brake down the accumulated sludge in order to find out how/why it is formed on the first place (chemical structure), and consequently, to figure out how to prevent its formation in the future. Another one would be to investigate reason(s) for filter clogging by identifying possible byproducts generated due to incompatibility of mixed lubricants, and so on and so forth. There are not many standard methods available, because there are no two lubricants that are the same (different additives packages, base oils). Some of the techniques, following the selective extractions, would require use of a scanner electronic microscope, GC, GCMS, ICP (combined with an acid digestion), XRF, laser particle counter, FTIR, ultracentrifuge, instruments and procedures for developing ferrograms, filtergrams (direct reading and/or analytical ferrography), identifying metallurgy, availability (and knowledge to use it) of wear debris atlas, etc., etc. It all depends on what you want to learn from analyzing debris from your filter.
