There are several good choices. If you're looking into medium oil, a great pearl that came out is the Storm Victory Road. It has a great core/cover, and from what I've seen, carries really well. I throw the Roto Grip Nomad Dagger on medium oil and even though its a hybrid solid/pearl, it has a really nice downlane motion and I love how it carries.
900 Global has 2 great balls out for medium conditions- the Favorite (solid) and the Long Shot (pearl). Both are really good choices. I did see a lefty throw a Long Shot a few weeks ago on the pair next to me. With the medium oil, it had good length and carried well. However, it did start to check too early once the lane broke down further. Carry was good from what I saw.
From Hammer, there is the Midnight Vibe which is designed for medium-dry conditions, or the Jigsaw Trap and the Hardcore Hammer Brawl for medium oil. I have yet to see any of these 3 thrown where I live.
Hope this helps to give you an idea of whats available.