Depends on what you want the ball to do. If you need it to be rolly, with a smooth type reaction, then a fresh coverstock would probably be best. If you need it to get downlane longer, then yes, the cover needs to pick up some oil. Really though, the ball is meant to be smooth, so you should choose a copperhead if you want more overall hook with backends, or choose the Swarm if you need more skid/flip. Make sure you always choose the right ball to begin with rather than buy the wrong type reaction and then try to manipulate it with layout or cover preperation.
RoB LaW
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I'm gettin' old, I'm hurtin', but I've got Brunswick balls.......Color me competitive.