I guess a lot of it depends on what you define as "medium" oil, and how you throw the ball. Since you don't have anything other than your speed listed, all I can do is give you options. Since I've had a chance to throw all of these balls (I love the Storm/Rotogrip Open Houses!), here's a rundown of some options from the Storm/Roto current equipment:
Rotogrip Grand Illusion (medium RG polished solid, very smooth, surprisingly good carry), Rotogrip Pluto (longer and just a little weaker than the Grand Illusion, but far from a light oil ball), Storm Hy-Road (solid/pearl hybrid coverstock, polished OOB, very versatile), and for the lighter side of medium, a polished Street Rod Solid is another good option. If you want something more angular for playing deeper lines, the Rapid Fire (polished solid) or Rapid Fire Pearl or T-Road Pearl are good options.
Or if you can find one (they have been discontinued), the Storm Spit Fire is one my favorite medium oil balls ever.
