No filtering at all.... personally thats crazy... Yes I too have heard of people draining the engine oil out of the engine, and pouring into the fuel tank w/o filtering.... crazy.... just crazy to me
Keep in mind that even if you pour straight unfiltered WMO or ATF into your fuel tank, you do have a filter between the tank and the IP, so your IP isn't seeing the same debris that you are dumping in your tank, which by the way was filtered by a oil pump if it is WMO. I filter to 10 microns with a filter bag and then it goes in the tank. I assume that my filter/water separator does the job from there.