My answer would be:
Heavy oily: The Arc. The most aggressive ball that I have ever thrown. After ninety games, I could pick up ten pins with it. I currently grow marigolds out of the finger holes.
Medium oil: The Revmaster. The best, most hooking, least expensive, medium performance ball that I have ever owned.
Bone dry:
That is a good question. I think any unaggresive ball with more ball speed is good. When they are this dry, you have to hit the ball less, increase your speed and play through the middle. That is why people with less bowling talent bowl well under this condition, while good or excellent bowlers struggle in this condition. If you are a good bowler, you basically have to try and forget everything that you have worked so hard to learn, when bowling in this condtion. Less hit, less fingers, less hand, more speed. That is why bowlers with half a hand bowl well under this condition. Pick a non-aggressive ball and go!
-Scott-
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When the tide of life is against you, and the current upsets your boat, don't waste those tears on what might have been, just lay on your back and float!