I asked the USBC to further define what constitutes a "foreign substance".  They responded with the following:

The prohibition on adding a foreign substance to the outer surface of the ball are specific to items a bowler may possess that they introduce to the outer surface of the ball.  It does not refer to the lane oil that the ball may pick up on its trek down the lanes or to anything that the ball may pick up through the actions of the pinsetter or ball returns.

So lane oil, grease, belt burns/waxy substances are not considered "foreign substances".  The ball can continue to be used with those substances on the surface and still be considered acceptable for use during competition.