First, I'd see if any of the plugs on that bank are oil fouled. This will isolate cylinder. Then if rings are good, like you said ( gaps placed correctly ), and there is no hole in the piston, and the bore is not egg shaped, I'd go after a leaky intake manifold gasket at that intake port, sucking oil out of the lifter gallery. If this is the case, there should be raw oil in the intake port. Worn out valve guides will cause it too. These things are what I'd look at first....