avabob is right, anybody that left their urethane ball out in the car all day could tell you that.
Many times I had my urethane balls in the car while at work and then go directly to the lanes. Open the bag and oil would be running off it.
Urethane balls absorb oil but at much slower rate than reactives
This is part of the reason why A lot of bowlers today don't see it today, because they don't use them all the time like we did back then (or keep them as long), so they don't really have time to get saturated with oil like they did back then.
Plus today bowlers clean their balls a bit more often than back then, giving them even less chance to absorb oil.
But they will absorb it and it can build up.