Havent tried all the newer urethane balls, but to me the blue hammer acts a lot like the old blue hammer, at least from a shell perspective. Also, the old urethane balls absorbed oil, partly because we kept them with much more surface than is common today. For example Blue Hammer, circa 1988, came at 500 grit box finish. Those balls definitely soaked up oil as can be attested by the fact that they typically changed from powder blue box to more of a sea blue/green with 30-50 games on them. My newer blue hammer actually seems to absorb less oil than my original simply because I keep it at 4000 wet sanded. Still subject to big time carrydown though.
Urethane and Polyester, are both much more susceptible to carry down than the resin balls of today. They push oil down very quickly, particularly with the high volumes that are used today. Resin balls also push oil, but they absorb more of it to lessen the amount carried down. However the big factor that helps negate the carry down impact is the chemical friction created by the resin additive. Resin balls will skid more easily through the head oil, conserving rotational energy. Then the resin additive makes them chain up and change direction more quickly when they come off the end of the buff, even when they do encounter some carrydown. You can often feel the effect of carrydown with resin on flatter patterns, but it doesn't last very long in the transition
