Most are for medium-light to medium-heavy.  Heavier than that and you're really looking at a solid particle, lighter and you're looking at a pearl resin.  I'd say that the Shift and Mystic (and the Mystic's predecessor, the RS-P) are for the heavier side of medium, especially with the shine knocked off.

SH

Mystic comes polished...if you knock the shine off, maybe take it down to 1000 abralon, trust me, it will hook.  The cover/core combo is quite strong.
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I am the University of Virginia's Bowling Team.  After all, one All-American is all you need.

I would say, OOB finish, Particle Pearls are for medium light to medium heavy> If you take some of the polish off and knock them down in abralon finish, then you can use them on heavy oil. But that's not what they are made for, and there are particle solids that are made for that type of condition. For what you want, the Mystic is a great, great choice. It is stronger than you think it would be and works well on medium/heavy conditions, and if you take the polish off of it, it will handle heavier volumns of oil. However, why not get a Horizon Solid instead? It's at 800 matte surface and can be polished easily as well.

Chuck

Fyi.., from my experience and from what I've read, as well. The Shift is a medium oil ball and is best suited for medium and medium-light. Anything heavier surface adjustments are needed - and it will also be a temporary solution.