I use a dual filter multi-pass setup where I circulate 4 gallons of oil through this setup for an hour or more at a time any number of times until I'm ready to use it. I mix it with fuel to lube the pumps and injectors. Have maybe $150 in it including the pump, filters, tank, hose, fittings, etc:
I think it comes out pretty darn clean for used oil. The filters lasted almost three years and filtered probably about 25 gallons of oil before I had to change them last fall. You should have seen the big globs of sludge that drained out from the large Fleetguard cannister when I did the filter change.