The oil ring is a completely different animal than the top two rings. Unlike the others, the tension of the oil ring is controlled by the radial thickness of both of the oil rails as well as the size of the gap in the expander ring. By manipulating the different parts of the oil ring to reduce the tension, Dorton has found a couple of horsepower on the dyno without harming oil control.
The radial thickness of the rings can vary by as much as 0.015 inch. This variance comes in different production runs, so you shouldn't see that much change in one pack of rings. But because the ID of the oil ring sits inside the expander, a ring with a greater radial thickness will have more tension than a ring with less radial thickness. This is true even if both rings have the same end gap.
Begin by separating out your rings according to thickness. Keep the thinnest and save the rings with the greater radial thickness for a later rebuild. You may need to go through a few packs to get a good set. If you cannot afford to purchase several sets of rings, see if your local engine builder would be willing to swap a few with you.
You can also check for variances in the gap in your expander rings. A properly sized expander ring will fit in the cylinder bore so that the ends butt without buckling the ring. You cannot gap an expander ring, but if you have one that has a gap when placed inside the bore, you can match it with scraper rings that have a greater radial thickness.
You can actually feel the difference in a well-matched set of low-tension oil rings. Install both oil rings with the expander on a piston and install it upside-down in a dry cylinder bore. Slide the piston up and down inside the bore and get a feel for the resistance. Now try the same thing with a set of oil rings with a greater radial thickness matched to the same expander ring and notice the difference.