Don't want to be the naysayer, but is there even Heavy Oil in Australia?
My advice is don't get caught up in hype around Heavy Oil balls. I don't think any brands make a Medium Oil ball that can't cover nearly any house condition or even most tournament patterns here.
Just because it rolls more, doesn't make it better. You also run the risk of your ball rolling out, or losing it's power.
Roto Grip and Storm balls are heavy rollers anyways so anything in the mid range with a well thought out layout should satisfy your needs.
A Totally Defiant should be more than enough for you. Anything more aggressive is a waste I think.