Just to clarify things:
"Rosswell" hardeness is really Rockwell hardness. This is simply a common methodology to measure hardness of a material. It deals with the depth that a diamond point will penetrate under a fixed load. A Rockwell "C" hardness of 60 to 62 is indicative of a carburized material and is "file hard" meaning a file is about the same hardness.
"Ostempford" is really austempered - a heat treat condition generally used for cast ductile iron. If you're really inquisitive, and want to be totally bored, go here:
www.appliedprocess.com/html/what_is.html
With a cast iron gearing, one member must be a different material than the other. This is because of the metallurgy of cast iron. Two cast gears together are basically like rubbing two pieces of sandpaper together. A bronze, steel, or composite gear should work - I would heed Rick's advise though on the composite. If designed correctly, the composite should be the least sensitive to any
oil problems - it could almost run dry. For proven performance, go with the steel or bronze.
Todd