FYI, as a test one time, I went through my normal procedure of wiping the ball surface off before every frame and after 30 games or more, doing a hot water/Dawn bath, then put the ball into a NuBall. No oil came out that I could see.
Another test, just wiped the ball off every frame, then after 30-50 games, gave the ball to pro shop with rejuvenator (don't know the brand) and had them run it through. When I picked the ball up a few days later, I asked how much oil came out and was told 'very little'.