For heavy oil and staying with the Storm - I would look into the current Roto-Grip line. The Hyper Cell has been very popular for heavy oil. The Sinister has interesting looks as well.
If you want to strictly go with Storm, I personally would wait until their next series of new releases. There must be a solid cover/heavy oil ball that will be in their Premier Line in the works. Personally, I am not a fan of the Zero or Byte, and I think most IQ's would fit in the med-heavy category as well.
It seems like you have the spare and light oil ball figured out. Remember that you should get equipment for need and not want. It seems like the Optimus and Assassin are both med-heavy pieces.
I do not have either, but the Assassin I think you should be able to play a bit heavier volume than the Optimus, but the Optimus you should be able to move in and it continue better as the lanes transition. Each has an advantage and disadvantage, you just got to decide which would fit you best.