Ok, what I'm going to say may sound nuts, but here ya go.
While I've never head of this problem on a Jeep engine, I have heard of this problem in rare instances on various Japanese engines. In some import engines, you can have a blocked or partially blocked PCV valve, and this can have an effect on a distributor that has a weak o-ring shaft seal in it. What happens is that when the engine is running at temperature, it can cause a tiny bit of oil saturated, hot crankcase gas to be forced past the o-ring and introduced into the inside of the distributor cap. Once this vapor is inside there, it will tend to condense on the first surface it encounters which is cooler. Because of the heat transfer properties of carbon, or the lack thereof, compared to the other materials inside the distributor, the button will be the first place the oil ends up. It cools, condenses, and voila, you end up with a bit of oil on the top of your rotor. Since it is then fouling your ignition system, you never have the opportunity to run the engine long enough for any significant amount of oil to show up inside your cap.
Unless you find another reason for this oil being there, you might want to make sure your PCV valve is functioning properly, and if the problem doesn't go away after that, you can reseal the distributor shaft.
