Ron,
Present day state of the art hydraulic oil filters use very fine fiber glass strands, around 1 micron in diameter, they are combined into a matrix that is the level of filtration that you may want, 1 to 40 microns and then they go thru a process that joins them together at every spot that they touch each other. This makes a very strong filtering media that can be manufacturesd to filter particles from .2 to 40 microns, and the size of the channel that the oil goes thru is very consistent so the filter efficiency is very high. Good filters have a filtration efficiency of over 99% single pass which mean that a one micron rated filter will capture 99 out of a 100 1 micron particles in the first pass. Automotive filters which are mostly paper cannot get to that efficiency and they have very large strands so the flow efficiency is low, i.e. high pressure drop thru the filter. The military and modern hydraulics have not used paper elements for over 30 years. Paper elements do have a place as the paper will adsorb water so paper elements are used to remove some free water in oil.
I believe that the very best place for the most efficient filter is on the return line from the scavenger pump, assuming that you have a dry sump oiling system. This placement prevents contamination of the oil tank and provides clean oil to the pressure pump. You can also put a filter on the outlet of the high pressure pump just to protect the engine from contamination that enters the oil tank when you open it and pour in "new" oil, which is quite dirty.
I believe that a good fiber glass 10 micron element, high flow rated, would be best for any automotive application. If you want to read about real oil filtration technology go to the Pall Filter site and look at their information. They are the leader in filtration.
Rex
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