I don't have the Neptune, but I have two Mystics and 1 Venus. Mystic #1 is laid out pin down, 4-1/2" from PAP, CG at 45 degrees, weight hole on VAL 1" below PAP. Mystic #2 is laid out pin up, 5-1/4" from PAP, with the CG at 45 degrees, no weight hole. Both are at box condition. Mystic #1 is much smoother, where Mystic #2 has a much harder arc.
The Venus is probably my favorite Roto-Grip ball. I have a label drill on it and the surface is 4000 Abralon + Beans' Secret Sauce. This is my go to ball on the THS where I bowl league. I may get another Venus and drill it pin up. This ball is much stronger than the numbers would indicate, but it rolls so smooth and predictable. The coverstock, like all Roto-Grip equipment, takes surface changes easily.
The Mystic is much stronger than the Venus and you can definitely justify having both.
For your heavy oil ball, the Quest is a good choice, but I would recommend taking a look at the Cell.
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