Well, is the Oberg really a low restriction filter? I have a couple of them. One is in a VW I raced, but it is in the pressure side before the external coolers. No issues there with the pressure.
The other is after the pump for my 915 spray oiling.
But I don't see where these are low restriction. They are a single, not too large (though they come in various sizes) flat mesh screen, as in the picture of course. Comes in a couple of mesh sizes, but none are very open. And they have a pressure relief bypass as well, or at least mine do - in one it hooks up with a switch to tell you if it is bypassing. Better late than never, I suppose.
I use a Canton Mecca filter on the scavange side before my front oil coolers. It doesn't have a bypass. And a System One pleated wire filter on the return between these coolers and the tank (front mounted). I, too, did that because of fears that engine explosions would foul the expensive oil coolers (were twin Continentals of some type, now two 964s), and mess up the tank too (though the one I now have comes apart for easy cleaning).
I also have a System 1 filter in place of the engine oil cooler. As the factory did on its race motors, and later does on the 964/993 (and the water coolers?).
That one, one would think, would prevent junk from getting into the bearings - if it got into the tank it could get into the pump (but if just babbit bits, ought not to hurt that?), and up through the initial pressure gallery and into the filter.
I like the System 1s because they are easy to take apart, inspect, and most of all, clean.
But a quick mental comparison with the Oberg tells me the area of screen on the larger of the two System 1s on the market is a lot greater that the largest Oberg. Though I haven't measured. And I wouldn't want to put even that in the all important suction line.
I wouldn't say that the return line to the oil pump is gravity fed. At least not solely gravity fed. Because the atmosphere is busy pushing the tank's oil into the spinning pressure pump, which is "drawing a suction," as they say. But its "suction" pressure is going to be limited to not much more than 15 PSI, no?
I thought I was going overboard upgrading to a tank return line from a -12 to a -16. Then the next full on track car I looked at had a -20! But the 16 does the job.
Maybe somene has quantified the flow restrictions imposed on various viscosities of oil from various filters?
I'm glad Green 912 hasn't had any problems with his nicely laid out system (mine, as you can see, is kind of a hodgepodge).
But I wouldn't put a filter on the tank return if you gave me one for free. Concentrate on stopping the crud before it gets to the tank. There is plenty of pressure in the scavange line to do this.