GoodEnough,
The stock oil pump is designed for a 100 to 200 micron filter (per TwoMany). The pressure with the stock filter is below 5 PSI and down to zero at idle, so I would think that you cannot go any finer with the stock oil pump. Even if you use a filter with a larger area, like a pleated paper filter, the pressure will go up on the pump side as the filter accumulates particles. The paradox here is that if you do use a finer filter, the pressure on the pump side may actually increase, but that just means that the flow has been reduced on the engine side of the filter, hence my suggestion to use Mikes high flow pump, but then you may encounter excessive flow at low temperatures which could hole the sump filter, so install a gauge to monitor the pressure when starting cold until the engine has warmed up.