Also, oil WILL NOT be filtered without the plug. The pressure in the vertical oil feed is constant. Any pressure causing oil to flow to the filter, would be cancled by the same pressure pushing back against filtered oil. Zero pressure differential = Zero flow.

Also, there is no severe restriction in the vertical oil shaft until after the RH main oil gallery feed. (except the small step where the plug is pressed in.)

If anything without the plug the oil pressure will be better at the sending unit because it doesn't have to make the four 90 degree turns to get to/from the oil filter, not to mention the restriction of the oil filter itself.

Now leave the rear plug out of the driver side oil gallery, and low oil pressure at idle is a tell tale sign it was forgotten.

(by the way, you might need to click "Attachment" to read these pictures better.)