I think it will all depend upon how old the ball is and how much oil has soaked into the coverstock.
A microwave works by passing microwave radiation with a large wavelength through the inner chamber. Usually anything (food) that would be put into a microwave would have water of fat inside. These molecules are electric dipoles that will constantly rotate(to try to align with the alternating electric field created by the waves)...this spinning is what creates the heat that you are accustomed to from a microwave oven.
Unless the materials in a bowling ball conduct electricity (which I'm not sure of) I don't think you will see much of a reaction
now if there is a lot of oil in the coverstock...
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University of Maryland 5th Year Senior
Mechanical engineering degree - May 2007
Then moving to the Norfolk/Chesapeake VA area
Looking for old Brunswick, Quantums and Storm NIB