Just to chime in on this, the Mutant, Freak and Super Freak are all discontinued. As for the other 5 you mentioned, the all handle different amounts of "heavy" oil. The Animal handles the most oil by far. Next would probably be the Phenom, then the Unleashed, then the AU. As for the Rule, I dont know where that will fit yet as I havent thrown one yet.
If you were to try to throw the Unleashed on a true heavy oil shot, the ball would not work very well. Just as if you tried to throw an Animal on a broken down heavy shot, it might not work as well as one of the others. That is why they have alot of "heavy oil" balls.
Plus, lets face it, ALOT of centers are putting down alot more oil on their lanes nowadays. Maybe not so much in the summer, but in the fall leagues around here, I see anything from a med/hvy shot to a flood on a weekly basis. Even the guys who crank the ball are playing a direct line!!
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"Why cant I just throw the ball the same way EVERY TIME??